Adi Shankaracharya taught Advaita philosophy. What was his belief on how one should see Shiva and Vishnu? Was he a Shaiva or a Vaishnava? Are Shiva and Vishnu same as per Him?
Shankaras Advaita is called Kevala Advaita ie. Only Advaita; there is no duality at all and any duality that is perceived is just due to ignorance. Here are some features of Shankara's philosophy:
1) Omniscience, omnipotence etc.. attribute in Lord is seen due to Avidya.
This point is made by Adi Shankara in Brahma Sutra Bhasyam 2.1.14. This point clearly separates Advaita from other philosophies like Vishistadvaita. In Brahma Sutra Bhasyam 2.1.14 Shankara says:
तदेवमविद्यात्मकोपाधिपरिच्छेदापेक्षमेवेश्वरस्येश्वरत्वं सर्वज्ञत्वं सर्वशक्तित्वं च, न परमार्थो विद्यया अपास्तसर्वपाधिस्वरुपे आत्मनि ईशत्रीशितव्यसर्वज्ञत्वादिव्यवहार उपपद्यते, तथा चोक्तम् - 'यत्र नान्यपश्यति नान्यच्छृणोति नान्यद्विजानाति स भूमा इति' यत्र 'त्वस्य सर्वमात्मैवाभूत्तत्केन कं पश्येत्' इत्यादिना च एव परमार्थवस्थायां सर्वव्यवहाराभावं वदन्ति वेदान्ता ।। 2.1.14
Hence the Lord's being a Lord, his omniscience, his omnipotence, &c. all depend on the limitation due to the adjuncts whose Self is Avidya; while in reality none of these qualities belong to the Self whose true nature is cleared, by right knowledge, from all adjuncts whatever. Thus Scripture also says, 'Where one sees nothing else, hears nothing else, understands nothing else, that is the Infinite' (Ch. Up. VII, 24, 1); 'But when the Self only has become all this, how should he see another?' (Bri. Up. II, 4, 13.) In this manner the Vedânta-texts declare that for him who has reached the state of truth and reality the whole apparent world does not exist.
Thus the above Bhasya makes it clear that the attributes of God like Shiva and Vishnu (like omniscience, omnipotence etc..) are seen due to Ignorance. These things do not exist at all. So, the Shiva-Vishnu debate doesn't exist in Shankaras philosophy at all. This also makes it clear that when Shankara defines highest thing using name of God, it clearly represents Nirguna Brahman.
In the same Bhasya Shankara also says:
Such texts are, 'In that all this has its Self; it is the True, it is the Self, thou art that' (Ch. Up. VI, 8, 7); 'This everything, all is that Self' (Bri. Up. II, 4, 6); 'Brahman alone is all this' (Mu. Up. II, 2, 11); 'The Self is all this' (Ch. Up. VII, 25, 2); 'There is in it no diversity' (Bri. Up. IV, 4, 25).--On any other assumption it would not be possible to maintain that by the knowledge of one thing everything becomes known (as the text quoted above declares). We therefore must adopt the following view. In the same way as those parts of ethereal space which are limited by jars and waterpots are not really different from the universal ethereal space, and as the water of a mirage is not really different from the surface of the salty steppe--for the nature of that water is that it is seen in one moment and has vanished in the next, and moreover, it is not to be perceived by its own nature (i.e. apart from the surface of the desert -; so this manifold world with its objects of enjoyment, enjoyers and so on has no existence apart from Brahman.
2) Shankara rejects philosophy of both Shaiva Aagams and PanchaRatra Aagams:
Those Vaishnavas, who adhere to Vedanta darshan, commented on Brahma Sutras by intrepreting Sutras in favour of PanchaRatra (Vaishnava) Aagams. Similarly , those Shaivas, who adhere to Vedanta darshan, commented on Brahma Sutras by intrepreting Sutras in favour of Shaiva Aagams.
However Shankara rejects both PanchaRatra Aagams and Shaiva Aagams.
Eg. Shankara rejects philosophy of Shaiva Aagams here as:
The Máhesvaras (Saivas) maintain that the five categories, viz. effect, cause, union, ritual, the end of pain, were taught by the Lord Pasupati (Siva) to the end of breaking the bonds of the animal (i.e. the soul); Pasupati is, according to them, the Lord, the operative cause.--Similarly, the Vaiseshikas and others also teach, according to their various systems, that the Lord is somehow the operative cause of the world. Against all these opinions the Sutra remarks 'the Lord, on account of the inappropriateness.' I.e. it is not possible that the Lord as the ruler of the pradhâna and
the soul should be the cause of the world, on account of the inappropriateness of that doctrine.
Similarly Shankara criticises PanchaRatra (Bhagvata) doctrine [ here ] as:
Moreover, manifold contradictions are met with in the Bhâgavata system, with reference to the assumption of qualities and their bearers. Eminence of knowledge and ruling capacity, strength, valour, and glory are enumerated as qualities, and then they are in some other place spoken of as Selfs, holy Vâsudevas, and so on.--Moreover, we meet with passages contradictory of the Veda. The following passage, for instance, blames the Veda, 'Not having found the highest bliss in the Vedas Sândilya studied this sâstra.'-- For this reason also the Bhâgavata doctrine cannot be accepted.
Thus, for this reason also, Shankara can be neither called Shaiva nor Vaishnava.
3) Shankara calls Narayana as the witness of all:
In the BrihadAranyaka Upanishad Bhasya 3.7.1 Shankara states:
देवताकार्यकरणस्य ईश्वरसाक्षिमात्रसान्निध्येन हि नियमेन प्रवृत्तिनिवृत्ती स्याताम् ; यईदृगीश्वरो नारायणाख्यः, पृथिवीं पृथिवीदेवताम्...
The body and organs of the deity of the earth are regularly made to work or stop work by the mere presence of the Lord as witness. Such an Iśvara, called Nārāyaṇa, who controls the deity of the earth....
4) Shankara calls Shiva as Sarvajna Ishwara:
While identifying the Uma Haimavati Devi in Kena Upanishad Bhasya, Shankara states:
अथवा उमैव हिमवतो दुहिता हैमवती नित्यमेव सर्वज्ञेनेश्वरेण सह वर्तत इति ज्ञातुं समर्थेती कृत्वा ताम् ।।
Or Uma is Haimavati as she is the daughter of Himavat. As she always lives with that Sarvajna Ishwara (Shiva), she knows the Brahman.
5) Shankara cites Lord Avimukta as highest:
In the Brahma Sutra Bhasya 1.2.32, Shankara cites a Shruti passage from Jabala Upanishad which states Highest Lord is Lord Avimukta (Shiva) and he resides in Varanasi (which I also discuss here)
आत्मनन्ति चैनं परमेश्वरमस्मिन्मूर्धचुबुकान्तराले जाबाला - " य एषोऽनन्तोऽव्यक्त आत्मा तं कथमहं विजानीयामिति ॥ स होवाच याज्ञवल्क्यः सोऽविमुक्त उपास्यो य एषोऽनन्तोऽव्यक्त आत्मा सोऽविमुक्ते प्रतिष्ठित इति ॥ का वै वरणा का च नाशीति ?
Moreover the Gâbâlas speak in their text of the highest Lord as being in the interstice between the top of the head and the chin. 'The unevolved infinite Self abides in the avimukta (i.e. the non-released soul). Where does that Avimukta abide? It abides in the Varanâ and the Nâsî, in the middle. What is that Varanâ, what is that Nâsî?
And at the last of that Bhasya he says:
तस्मात परमेश्वरो वैश्वानर इति सिद्धम् ।
Thus it is proved that Vaishvanara is the Parameshwara.
6) Shankara in Vivekchudamani:
In the 494th verse of Vivekchudamani Shankara states:
पुरान्तकोऽहं पुरुषोऽहमीशः ।
निरीश्वरोऽहं निरहं च निर्ममः ॥
494. I am Narayana, the slayer of Naraka; I am the destroyer of Tripura, the Supreme Being, the Ruler; I am knowledge Absolute, the Witness of everything; I have no other Ruler but myself, I am devoid of the ideas of "I’ and "mine".
This also shows Shiva-Vishnu Abheda philosophy of Shankara.
7) Shankara while commenting the name 'Rudra' in Vishnu Sahasranaamam:
There is also a name 'Rudra' in Vishnu Sahasranamam and Shankara comments it as:
रुर्दुःखं दुःखकारणं वा, द्रावयतीति रुद्रः रोदनाद् द्रावद्वापि रुद्र इत्युच्यते ।
'Ru' means suffering or cause of suffering, and thus Rudra is the remover of suffering. Who ends the crying of samsara he is Rudra.
And then he cites a verse from Shiva Purana stating it:
'रुर्दुःखं दुःखहेतु वा तद्रावयति यः प्रभुः ।
रुद्र इत्युच्यते तस्माच्छिव परमकारणम् ।।'
"Ru means suffering and cause of suffering and Lord is the remover of that. Thus he is called Rudra and thus Shiva is the Parama Kaaranam (Supreme cause)."
Interestingly while commenting on names of Vishnu in Vishnu Sahasranaama he cites a verse from Shiva Purana which holds Shiva as Parama Kaarana. Thus it clearly shows that Adi Shankara held no difference between Hari and Hara. Shankara also cites a verse by Maheswara in Harivamsha in his Bhasya which states that name of Vishnu also belongs to Shiva.
नामानि तव गोविन्द यानि लोकेमहान्ति च ।
तान्येव मम नामानि नात्र कार्या विचारणा ।। (Harivamsha 3.88)
Oh Govinda your names which are famous in this world, these are also my names there is no doubt in it.
So, actually Vishnu Sahasranaama Bhasya written by Adi Shankara is also Shiva Sahasranama Bhasya.
8) Shankara cites statement of Sri Krishna:
Shankara in Vishnu Sahasranama Bhasya cites statement of Lord Krishna from Vishnu Purana which was told by Lord Krishna to Lord Shiva during Banasur event:
त्वया यदभयं दत्तं तद्दत्तमग्विलं मया ।
मत्तो विभिन्नमात्मानं द्रष्ट्रं नार्हसि शंकर ।।
योऽहं स त्वं जगच्चेदं स देवासुरमानुषम् ।
अविद्यामोहितात्मानः पुरुषा भिन्नदर्शिन ।।(Vishnu Purana 5.33.47-49)
That Abhaya which was given by you was also given by me. Oh Shankara do not see yourself different from me. That which is me the same is you, this entire creation, Devas, Manyushyas and Asuras. Only those who are deluded by Avidya see me and you as different.
Thus Shankara expounds those who see difference (especially between Hari and Hara) are "Avidya Mohita Atman" (those deluded by Avidya).
9) Shankara's warning to those who see difference:
Shankara in his Vishnu Sahasranama Bhasya explicitly states seeing Trimurtys (especially Hari and Hara) as different is a great sin. Shankara cites statement of Maheswara from Bhavisyottar of Harivamsha as:
विष्णोरन्यं तु पश्यन्ति ये मां ब्रह्माणमेव वा ।
कुतर्कमतयोमूढा पश्यन्ते नरकेष्वध ।।
ये च मूढा दुरात्मानो भिन्नं पश्यन्ति मां हरे ।
ब्रह्माणं च ततस्तस्माद् ब्रह्महत्यासमं त्वधम् ।।
He who sees myself and Brahma different than Vishnu, such fools surely fall in hell. That Durãtmãn who sees myself different from Hari does a sin equivalent to BrahmaHatya.
Similarly Shankara also states statement of Maheswara as:
आवयोरन्तर नास्ति शब्दैरर्थैर्जगत्त्रये ।। (Harivamsha 3.88)
There is no difference between you and me neither by Shabda (word) nor by meaning (artha).
Thus it's clear that Shankara taught Trimurty Abheda philosophy. From the above statements we can know that Shankara holds those who see difference between Hari and Hara are Avidya Mohita Atman (Deluded by ignorance),Kutarkavadi (Who generates nonsense logic),Mudha (Who is fool), Narakeshvada (Who goes to Naraka), BrahmaHatya Sama (Who does sin equal as BrahmaHatya)
All these are not self made claims of Adi Shankara. Shankara cites these verses or passages from Harivamsha Parva and Vishnu Purana.
One of your questions is "Was Adi Shankara a devotee of Lord Shiva or of Lord Vishnu?".
Now,while this can not be ascertained definitively but we can surely get some hints.
The concluding verse of Ganga Stotram is :
गङ्गास्तोत्रमिदं भवसारं वाञ्छितफलदं विमलं सारम् । शङ्करसेवकशङ्कररचितं पठति सुखी स्तव इति च समाप्तः ॥१४॥
Ganggaa-Stotram-Idam Bhava-Saaram Vaan.chita-Phala-Dam Vimalam Saaram | Shankara-Sevaka-Shankara-Racitam Patthati Sukhii Stava Iti Ca Samaaptah ||14||
Meaning: 14.1: (Salutations to Devi Ganga) This Ganga Stotram is the true substance in this Samsara, giving desired Fruits, and is the essence of Purity, 14.2: This hymn is composed by Shankara (Adi Shankaracharya), the servant of Shankara (Shiva); those who read it will be filled with Joy; thus ends this Stava (Hymn) (with good wishes for all).
So, here Adi Shankara declares himself as to be a "Shankara Sevaka" or a servant of Bhagavan Shankara(Lord Shiva).So,this indicate that he might be a devotee of Lord Shiva.
Similarly,the concluding verse of the Annapoorna Stotram is:
माता च पार्वती देवी पिता देवो महेश्वरः । बान्धवाः शिवभक्ताश्च स्वदेशो भुवनत्रयम् ॥१२॥
Maataa Ca Paarvatii Devii Pitaa Devo Maheshvarah | Baandhavaah Shiva-Bhaktaash-Ca Svadesho Bhuvana-Trayam ||12||
Meaning: 12.1: (Salutations to Mother Annapoorna) My Mother is Devi Parvati, and my Father is Deva Maheswara (Shiva), 12.2: My Friends are the devotees of Shiva, and my Country is all the Three Worlds (Whose Lord is Shiva-Parvati).
This verse also can be taken as to indicate that Sri Adi Shankara was a devotee of Shiva-Shakti.
But in any case ,a devotee of whomsoever,Adi Shankara never discriminated between Shiva and Vishnu as explained in Tezz's answer.
And,for that reason,he promoted the Panchayatana Puja system in which all the 5 major Hindu Deities(Shiva,Vishnu,Devi,Ganapati and Surya) are worshiped together with equal respect.
Adi Sankaracarya is an avatara of Shiva, as stated in Padma Purana (6.236.5-12), Kurma purana (1.30.33-34). Shiva Purana also quotes the Supreme Lord ordering Lord Shiva: "In Kali-yuga mislead the people in general by propounding imaginary meanings from the Vedas to bewilder them." How the Supreme Lord ordered Lord Shiva to appear in Kali-yuga to delude atheists and produce a philosophy and texts to hide the Supreme Being, and to make Shiva seem superior, is explained in the Padma Purana (6.71.89-116). Thus, we find in these verses confirmation that it was Lord Shiva who appeared in the Age of Kali as Shankaracharya at the request of the Supreme.
So, here we can see that he is incarnation of Shiva, but on other hands, in Bhagavata Purana, we can find that Shiva is the greatest devotee of Krishna, i.e. Vishnu.
In Bhagavata purana, 1.3, there is the list of Bhagavan's avataras, and 1.3.28 states... "ete camsa kalah pumsah, krsnas tu bhagavan svayam"... which means, "...mentioned avatars are plenary portions and portions of the plenary portions, but Krsna Himself (svayam) is Supreme God (bhagavan) for shure (tu)"...
Shiva is a vaishnama, so it is easy to conclude that Adi Sankara was Vaishnava as well with special mission - in the Padma Purana (6.236.5-12) Shiva explains to his wife, Parvati, that he will appear in the age of Kali to proclaim that the Buddhist doctrine is a false religion and illusory. He also said that he would propound the mayavada or impersonalist philosophy, emphasizing the indefinable nature of the Brahman, the great, impersonal spiritual force. He explained, "The philosophy of Maya (mayavada) is a wicked doctrine and is pseudo-Buddhist. In the form of a brahmana, I proclaim this doctrine in Kali-yuga. It makes the words of the holy Vedic texts meaningless and is condemned in the world. In this doctrine it recommends giving up one's duties of life [in order to be free of karma], which is said to be religiousness for those who have fallen from their duties. I will propound the identity of the Supreme Soul and the individual soul to be the [one and the same] Brahman in nature, without qualities. O goddess, I have conceived this mayavada (impersonalist) doctrine, which resembles a purport of the Ve das, for deluding people in this age of Kali [to mislead them toward atheism by denying the personal form of God]."
The Kurma Purana (1.30.33-34) states: "In Kali-yuga, Shankara, Nilalohita, will incarnate for the purpose of establishing rites of the Shrauta [Vedic] and Smarta [based on Smriti scriptures], with the desire for the welfare of his devotees. He will teach his disciples the knowledge of Brahman." The Shiva Purana also quotes the Supreme Lord ordering Lord Shiva: "In Kali-yuga mislead the people in general by propounding imaginary meanings from the Vedas to bewilder them." How the Supreme Lord ordered Lord Shiva to appear in Kali-yuga to delude atheists and produce a philosophy and texts to hide the Supreme Being, and to make Shiva seem superior, is explained in the Padma Purana (6.71.89-116). Thus, we find in these verses confirmation that it was Lord Shiva who appeared in the Age of Kali as Shankaracharya at the request of the Supreme.
Furthermore, he starts his commentary on Bhagavad gita with following statement:
narayanah paro 'vyaktat
which means that Narayana (Vishnu, Krsna) is ABOVE manifested world, he existed before manifestation, which is a bullet in the eye of every advaita follower. Advaita school teaches that Brahman, when manifested as avatara in material world, it assumes the material body, so Brahman incarnations had material body. But here we can see that Narayana WAS EXISTING before the manifestation and therefore He cannot have material body.
Adi Sankara also wrote a poem Bhaja Govindam, "A Hymn to Govinda - Krishna", and at the very first verse, he states ..."Worship Govinda, Worship Govinda, Worship Govinda. Oh fool! Rules of Grammar will not save you at the time of your death", calling his own disciples and lineage (parampara) mudha, fools and to worship (bhaja) Govinda, i.e. Krsna. He did not say worship (bhaja) Brahman...
To conclude, he was a vaishnava.
Adi Śankarāchārya was a Dashanami Shaiva who worshipped different gods (especially Lord Vishnu because his kuladevata and pradeśa devata was Krishna), Adi Śankarāchārya's real view regarding this hierarchy of gods is not yet decoded successfully by anyone till date. However modern Advaitins and even current heads (Jagadgurus) of different amnāya peethams (especially Sringeri's Sri Bharati tirtha swamigal) urge for Hari Hara abedha mindset among youth in the name of Adi Śankarāchārya, but it's very clear from Adi Śankarāchārya's authentic writings like Brahma sūtra bhāshya or Upanishad bhashya that all gods (devatas) are not equal even though philosophically the 'soul' which is pure consciousness is ONE and absolute.
He, in his commentary on Brahma sutras 1.2.17 makes it very clear that Surya (Sun god) cannot be worshipped as supreme god which goes against to Śrutis, rather the supreme lord who's the antaryami of Surya (Sun) is to be worshipped keeping sun as a medium, but today's Smārthas or Advaitins say Adi Śankarāchārya propounded a Shanmatha system of worship in which Surya is also worshipped as Supreme Brahman.
There's an allegation by few Vaishnavas that Adi Śankaracharya held only Vishnu as Saguna brahman and not anyone else (hence he's a Vaishnava) but that allegation goes upside down as Adi Śankarāchārya himself in his Brahma Sutra bhashya states:
Since in another Upanishadic text a specific statement is made thus, "Then a being created from the mind (of Brahman, i.e. Hiranyagarbha) comes and conducts them to the worlds of Brahman. They attain perfection and live in these worlds of Brahman for a great many superfine years", therefore it can be understood that the path is related to the conditioned Brahman only. For it is improper to use the plural number (in "worlds") in the case of the supreme Brahman whereas this plural number quite befits the conditioned Brahman (Saguna brahman), since there can be such a thing as difference of states in It. Even the Upanishadic use of the word "world", constituting a place of experience with its multiple aspects, fits in well with a conditioned entity, whereas in the other case (of the absolute Brahman) the word can be used only in a figurative sense as in such texts, "Oh Emperor, this is Brahmaloka (the world that is Brahman Itself. Again, to speak in terms of a container and a thing contained (as in, "In those worlds of Brahman") hardly fits in with the supreme Brahman. Hence this
escorting relates to the conditioned Brahman alone.
--(Śankara bhashya 4.3.8)
There's a very rarely known work called Prapancha Sara tantra penned by Ādi Śankarāchārya whose authenticity is unquestionable as it has Vivarana (commentary) by his own direct disciple Padmapādāchārya and is also quoted by ancient Advaitins of 12th and 13th centuries CE like Nārayanāśrama, Amalānanda etc: In that particular work Ādi Śaṅkarāchārya adores Lord Ganesha as supreme cause of the universe.
bijapūrgadhekśukārmuka rujāchakrābjapāshopthal vrīhāgrasvaviśānaratrakalāśaprodhyātkarāmbōruhū dhyēyō vallabhāya sapthakāryaśistō jvaladbhūshayaha viśvōtpattivipattisaṁsthikāro vignyō viriśtārdhaha ||
The adorable strongest deity who's undefeatable and indestructible, who's beyond time, the protector of beings who take his refugee is the lord of all seven functions of the universe, he rules the waters and from him evolves the universe and finally dissolves to him, he's the destroyer of all vignās (obstacles) the supreme godhead Gaṇeśa. (Prapancha Sara tantra 17.16)
Adi Śankaracharya worshipping Ganesha as supreme saguna Brahman denotes crystal clearly that Adi Śankarāchārya considered multiple godheads as saguna brahman, even Skanda (Karthikeya) as in the very same work Skanda is adored as saguna brahman.
Adi Śaṅkara bhagavatpāda on Shiva and Vishnu, Adi Śankarāchārya identifies Shiva as 'Avimukta of Varanasi' who's worshipped by Jabalas as supreme Lord in his commentary on Brahma sutras.
Moreover the Jâbâlas speak in their text of the highest Lord as being in the interstice between the top of the head and the chin. 'The unevolved infinite Self abides in the avimukta (i.e. the non-released soul). Where does that Avimukta abide? It abides in the Varanâ and the Nâsî, in the middle. What is that Varanâ, what is that Nâsî? Therefore the supreme lord (Avimuktā/Shiva) alone is the Vaishvanara (Cosmic Brahman). (Śaṅkara bhāśyam 1.2.32)
He says meditation on Lord Shiva as supreme non-dual lord as practiced by Jabalas is the highest Advaitic (Vedantic) mediation whereas mediation on images of Viṣṇu is limited only upto gouna bhakti (hardcore dualistic worship).
The supreme 𝗟𝗼𝗿𝗱 𝗣𝗮𝗿𝗮𝗺𝗲𝘀𝗵𝘃𝗮𝗿𝗮 is to be realized as the supreme absolute reality just like the way how 𝗝𝗮𝗯𝗮𝗹𝗮𝘀, 𝘄𝗵𝗶𝗹𝗲 𝘀𝗽𝗲𝗮𝗸𝗶𝗻𝗴 𝗮𝗯𝗼𝘂𝘁 𝘀𝘂𝗽𝗿𝗲𝗺𝗲 𝗣𝗮𝗿𝗮𝗺𝗲𝘀𝗵𝘃𝗮𝗿𝗮 address him as the eternal absolute Brahman the self, that oh 𝗘𝘁𝗲𝗿𝗻𝗮𝗹 𝗕𝗵𝗮𝗴𝘄𝗮𝗮𝗻 though you appear deity (Devata) thou indeed art me (𝗧𝗮𝘁 𝘁𝘄𝗮𝗺 𝗮𝘀𝗶), I indeed am thee (𝗮𝗵𝗮𝗺 𝗯𝗿𝗮𝗵𝗺𝗮𝘀𝗺𝗶), As a matter of fact, the Vedic texts make us understand Brahman as absolute The self is the supreme absolute, that absolute supreme self is the only reality. "That is Truth, that is the Self, and That thou art". As for the argument that on the 𝗮𝗻𝗮𝗹𝗼𝗴𝘆 𝗼𝗳 𝗮𝗻 𝗶𝗺𝗮𝗴𝗲 𝗯𝗲𝗶𝗻𝗴 𝗩𝗶𝘀𝗵𝗻𝘂 𝘁𝗵𝗶𝘀 𝗶𝘀 𝗼𝗻𝗹𝘆 𝗮 𝗺𝗲𝗱𝗶𝘁𝗮𝘁𝗶𝗼𝗻 𝘄𝗶𝘁𝗵 𝘁𝗵𝗲 𝗵𝗲𝗹𝗽 𝗼𝗳 𝗮𝗻 𝗶𝗺𝗮𝗴𝗲 𝘄𝗵𝗶𝗰𝗵 𝗶𝘀 𝗮𝗰𝘁𝘂𝗮𝗹𝗹𝘆 𝗶𝗻 𝘁𝗵𝗲 𝗰𝗮𝘀𝗲 𝗼𝗳 '𝗜' 𝗶.𝗲 𝗶𝗻 𝘁𝗵𝗲 𝗺𝗼𝗱𝗲 𝗼𝗳 𝗲𝘅𝗶𝘀𝘁𝗲𝗻𝗰𝗲 𝗼𝗳 𝘀𝗲𝗰𝗼𝗻𝗱 𝗲𝗻𝘁𝗶𝘁𝘆 𝗵𝗲𝗻𝗰𝗲 𝗶𝘁𝘀 𝗶𝗺𝗽𝗿𝗼𝗽𝗲𝗿 𝘁𝗼 𝘃𝗶𝗲𝘄 𝗶𝘁 𝗶𝗻 𝘁𝗲𝗿𝗺𝘀 𝗼𝗳 𝗮𝗯𝘀𝗼𝗹𝘂𝘁𝗲 𝘂𝗻𝗶𝘁𝘆.
Śaṅkara bhāśyam 4.1.3
Hence, in it's clearly notable that in Śaṅkara's system of Advaita, non dual mediation on Shiva is given much importance than gouna bhakti on Lord Viśṇu, thereby establishing clear difference between worship of Shiva and Vishnu.
In Krama mukti section of Brahma sutras 4.3 Adi Śankarāchārya makes it very clear that Viśnu who is worthy of worship as saguna Iśvarā is bounded by 'Sattva upādhis' hence an aupādhika devata, but Sureśvarāchārya (direct disciple of Adi Śankarāchārya himself) in his Manasollasa Vārtika explains that Parameshvara Shiva is beyond all Upādhis and is absolute Brahman by whose power Brahma, Vishnu and Rudra shine.
He in his Prapancha Sara tantra also quotes a verse from Kalika purana which glorifies Sharabha avatar of Shiva as destroyer of Varaha avatara of Vishnu, but on contrary even though there are several hymns in the very same work which are subjected to adoration to Vishnu, none of them put Vishnu as equal to Shiva.
Padmapādāchārya (another direct disciple of Adi Śankarāchārya himself) also has penned a work called 'Sri Panchakshari Vidya' which is a commentary on 'Shiva Panchākśara stotram' of Adi Śankarāchārya in which he establishes few facts (which are rarely known nowadays) about Shiva in Advaita Vedanta:
Shiva is the supreme Nirguna Brahman himself who appears as limited (as pradeshamātra) only for the sake of devotees.
Shiva is the supreme Pranava Omkara and the pure consciousness.
Hence, in Advaita system of Vedānta Shiva have more prominence as a godhead than Vishnu, due to increase of Vaishnava agents in Internet and neutral mindset of Advaitins such truths are very deeply hidden and are not entertained even though they are absolutely genuine and informative.
Adi Shankaracharya was not any Shanmata Sthapaka l or the one who implemented secularism by saying "All are same, what's in the name", Multiple Saguna brahmas etc: though the Advaita Vedanta focusses on single supreme truth (Brahman) at the highest form of reality (Paramarthika), in worldly level (Vyavaharika and Prathibashika), Jivatmas are those who are the devotees of Supreme Brahman and are affected by Maya which is illusory.
Adi Shankaracharya was a Ṣhaiva who propounded Dashanami Sampraḍāya and not any Smartha sampraḍāya as current day Advaitins proclaim. The Current day Advaitins are highly influenced by later practices which were mixed with pure Advaitic system. For example today's followers of Adi Shankaracharya believe that he implemented 6 deity equality worship which includes Shiva, Vishnu, Shakti, Ganesha, Surya, but this has no evidence from any of Adi Shankaracharya's authentic works, instead multiple contraries can be found out.
Adi Shankaracharya in his Brahma sutra bhashya 1.2.17, openly says that Surya (Sun) can never be supreme lord.
Selfhood cannot be ascribed to the sun, on account of his externality (parâgrûpatva). Immortality,&c. also cannot be predicated of him, as Scripture speaks of his origin and his dissolution. For the (so-called) deathlessness of the gods only means their (comparatively) long existence. And their lordly power also is based on the Parameshvara and does not naturally belong to them; as the mantra declares, 'From terror of it (Brahman) the wind blows, from terror the sun rises; from terror of it Agni and Indra, yea, Death runs as the fifth.'--Hence the person in the eye must be viewed as the Parameshvara only.
Also Adi Shankaracharya in his Mundaka Upanishad bhashya directly states that Vishnu is a jivatma which can be viewed here in detail, still I don't understand how some people (especially Sri Vaishnavites) are surviving in a dark utopia that Adi Shankaracharya embraced their bogus and non-vedic Vaishnavism.
Two among the Shanmata devaṭas has been rejected by Adi Shankaracharya openly with direct statements,by this we can understand that, lack of proper exploration or mindset to understand the actual essence of Advaitic philosophy has prevented people from knowing or rather digesting these hidden facts.
Adi Shankaracharya accepts Bhagavan Sharabha.
In Prapancha Sara tantra 25.3 Adi Shankaracharya has accepted one of the most controversial Shaivite aspect, i.e Lord Sharabheshvara liberating Lord Narasimha. T By just listening into this particular story, Vaishnavas get offended and start backfiring, but Adi Shankaracharya who's often considered as most secular has directly accepted this.
Adi Shankaracharya says Vishnu bhakti is limited to Gauṇaṭvam
In 4.1.3 of Śaṇkara bhashya, we can clearly observe that Adi Shankaracharya strongly believed in supreme non dual worship of Parameshvara (Shiva) as described by Jâbâlās in their Shruti, and also that worshipping gods like Vishnu in Murti form is just Gauṇa bhakti (Worship of act/speech).
Adi Shankaracharya says Brahma, Vishnu, Indra etc: attains death.
If there's anyone authentic stotram being composed by Adi Shankaracharya, let it be Saundarya Lahari alone. Saundarya Lahari is the only highly authentic work of Adi Shankaracharya as it's been referred back in 10th century itself by one of the famous Tamil poet called Kaviraja Panditar. Remaining works like Bhaja Govindam, Shivananda lahari etc: have no evidence or proper referential authenticity.
The 26th mantra of Saundarya Lahari sums everything very clearly.
O! Faithful Consort of Sadāshiva! At the time of great dissolution, Brahmā, Viṣṇu, Yama, Kubera, Indra and other gods get destroyed. But you alone enjoy with your Consort Shiva (Supreme Lord), oh supreme mother.
So it's very clear that Adi Shankaracharya was Shiva-Shakti abedha vadi and not Shiva-Vishnu abedha vadi.
To conclude, Adi Shankaracharya was a Dashanami Shaiva who ascribed to supremacy of Lord Sadasiva which is the supreme vedic path and also strongly believed that Vishnu is a Samashti Jivāṭma Devata who attains death during ultimate dissolution caused by Mahakala.
Vāraṇasi pura paṭim bhaja Viśwanaṭham
One Verse from Adi Shankara's Prashnottari Ratna malika alone is enough to answer this question.
But Since some other Answers have mentioned Prapanchasara Tantra, I'll be corroborating my point by citing from there as well.
In his magnum opus Prapanchasara Tantra (which is authenticated by several acharyas such as Amalananda, Vidyaranya, Sayanachara etc.), Shankara has eulogised all Shiva, Vishnu, Ganapati and Devi as the Jagatkaarana.
Relevant Evidences are shared below.
- The very first shloka of the work is a mangalacharana on Devi Sharada (Saraswati) who is glorified as the source as well as the substratum of the whole world.
- Vishnu Stuti From Chapter 21 of the Prapanchasara. Vishnu here is glorified as paramatman and the one who assumes the forms of the trimurti.
- Shiva stuti from Chapter 27 of the prapanchasara. As is the case with the Vishnu stuti, It's Bhagavan Shiva who is glorified here as the one manifesting as the Triad-Brahma, Vishnu and Shiva.
- The 26th chapter is on the Worship of Bhagavana Dakshinamurty.At the End of the Chapter, It's Stated that his worship makes one a great kavi and a knower of Vedanta who is bestowed with Advaita-Vidya and attains Shaiva pada .
- The 11Th chapter has a Beautiful stuti on Devi Bhuvaneshvari. Although the stuti focuses on her aspect as prakriti, her ultimate nature as the Antaratma chaitanya of all is also stated.
- 17Th Chapter of the Prapanchasara is on the of Ganapati. Sri Ganesha here is Glorified as Vishvotpatti-vipatti-samsthiti kara, suggesting him being glorified here as Jagatkaarana Brahman as well.