Vishnu as a greater god because.:
The Drona Parva of the Mahabharata and the Harivamsa reveal the boon given by lord shiva to lord vishnu after 66000 celestial years of penance as thus:
matprasādān manuṣyeṣu devagandharvayoniṣu aprameyabalātmā tvaṃ nārāyaṇa bhaviṣyasi ( Mahabharata 7 : 172: verse No. 74 ).
By the grace of me you will become Nārāyaṇa with unending strength among the deities, humans and Gandharvas.
kaś cit tava rujaṃ kartā matprasādāt kathaṃ cana api cet samaraṃ gatvā bhaviṣyasi mamādhikaḥ ( Mahabharata 7 : 172: verse No. 78 ).
“Through my grace, none shall ever be able to cause thee pain by the weapon of thunderbolt or with any object that is wet or dry, or with any mobile or immobile thing. Thou shalt be superior to myself if thou ever goest to battle against me.'”
This was again confirmed by Lord Siva to Lord Krishna before Lord Krishna was about to appear in battle against Vanasura as stated in the Harivamsa.:
yathA mainAkamAshritya tapastvamakaroH prabho | tathA mama varaM kR^iShNa saMsmR^itya sthairyamApnuhi ||2-74-37.
O lord ! O kR^iShNa! As you did penance staying on mainAka, you received a boon from me. Be firm in your mind, remembering that boon.
avadhyastvamajeyashcha mattaH shUratarastathA | bhavitAsItyavochaM yattattathA na tadanyathA || 2-74-38.
You can not be killed, you can not be conquered, you will be more valiant than me. All this will happen as told by me. None will be able to change this. Otherwise, who else is able to dare appear before Lord Siva in the battlefield when He holds His trident?
So, it was Bhagawan Shiva only who gave Lord Vishnu the boon to become superior.
Shiva Puja is the greatest because.:
“tato ‘rjunaḥ prītamanā vavande vṛṣabhadhvajam |dadarśotphulla nayanaḥ samastaṃ tejasāṃ vidhim ||” (Mahabharata Drona Parva 7:57:60)“taṃ copahāraṃ svakṛtaṃ naiśaṃ naityakam ātmanaḥ |dadarśa tryambakābhyāśe vāsudeva niveditam ||” (Mahabharata Drona Parva 7:57:61)“.
Sanjaya said, ‘Then Partha, with a cheerful soul and joined hands and eyes expanded (in wonder), gazed at the god having the bull for his mark and who was the receptacle of every energy. And he beheld the offerings he made every night to Vasudeva lying by the side of the (lotus feets of the) Three-eyed deity”. (Book 7, Section LXXXI in translation by K.M. Ganguly).
According to $Nirvana Upanishad,* how a yogi achieves Nirvana aka Moksha is given.:
The (worship of) gods named Vishnu, Brahma and a hundred others culminates (in Brahman). (Verse 29).
And Who is that Brahman after oneness with whom a yogi gets nirvana.:
परापवादमुक्तो जीवन्मुक्तः | शिवयोगनिद्रा च ।
Liberated while alive, as they are freed from denial of the highest (Brahman). The oneness with Siva is their sleep. (Verse 24-25).
I hope this clarifies all your queries. Prd..