Yes, Krishna is really the only ultimate God. This is the conclusion of all scriptures and seen in many scirptures like Bhagavata Purana, Vishnu Purana, Gita etc..
Krishna say in Gita (7.7)
mattaḥ parataraṁ nānyat kiñcid asti dhanañ-jaya mayi sarvam idaṁ
protaṁ sūtre maṇi-gaṇā iva
O conqueror of wealth, there is no truth superior to Me. Everything
rests upon Me, as pearls are strung on a thread.
Gopala Tapani Upanishad says:
sac-cid-ananda-rupaya kRsnayaklista-karine namo vedanta-vedyaya gurave
I offer obeisances to Krsna, the form of eternity, knowledge, and
bliss, whose every act is wonderful, who is the object of knowledge
identified by the Vedanta, and who is the guru, the witness present
in the intelligence.
One may say: “But one can’t conlude that Vishnu is supreme, because there is a variety of opinions expressed in the various Puranas uttered by Vyasa, where Brahma and Siva are also declared to be supreme.”
But supremacy of Vishnu/Krishna is conclusion of the scriptures.
Thus in Padma Purana, Patala Khanda, 97.26 it is said:
vyāmohāya carācarasya jagataste te purāṇāgamāstāṃ tāmeva hi devatāṃ
paramikāṃ jalpaṃtu kalpe vidhau siddhāṃte punareka eva bhagavānviṣṇuḥ
samastāgama- vyāpāreṣu vivekināṃ vyatikaraṃ nīteṣu niścīyate
व्यार्मोहाय चराचरस्य जगतस्ते ते पुराणागमास्तां तामेव हि देवतां परमिकां
जल्पंतु कल्पे विधौ सिद्धांते पुनरेक एव भगवान्विष्णुः समस्तागम-
व्यापारेषु विवेकिनां व्यतिकरं नीतेषु निश्चीयते
For bewildering the living entities of the universe, let the Puranas
and other scriptures speak, until the end of the kalpa, about various
“supreme” devatas. However, in conclusion, Lord Vishnu alone is
discerned in all the scriptures and in all conduct through harmonizing
all statements with intelligence.
For bewilderment of the universe composed of moving entities such as men and devatas and non-moving entities such as presiding deities of mountains, let the Puranas and other scriptures proclaim devatas such as Brahma and Siva to be supreme till the end of the kalpa. However, since there is a conclusion reached by the Brahma sutras and its commentary in the form of the Bhagavatam, through discrimination by use of direct and indirect meaning of the words, the form of Vishnu composed of unobstructed knowledge and bliss is determined to be supreme.
Thus we see in Bhagavatam (1.18.21):
athāpi yat-pāda-nakhāvasṛṣṭaṁ jagad viriñcopahṛtārhaṇāmbhaḥ seśaṁ
punāty anyatamo mukundāt ko nāma loke bhagavat-padārthaḥ
Who else would be worth the position of carrying the name of Supreme
Lord[Bhagavan] besides Mukunda [Lord Krishna as the one granting liberation]
from whose toenails the water emanated [of the Ganges] that was
collected by Brahmâjî and purifies Lord S'iva and the entire universe?
The Vishnu Purana also says that the term Bhagavan(used to literally indicate supreme) belongs to Lord Vasudeva alone. When used for others, it is used in a different sense:
The word Bhagavat is a convenient form to be used in the adoration of
that supreme being, to whom no term is applicable; and therefore
Bhagavat expresses that supreme spirit, which is individual, almighty,
and the cause of causes of all things. The letter Bh implies the
cerisher and supporter of the universe. By ga is understood the
leader, impeller, or creator. The dissyllable Bhaga indicates the six
properties, dominion, might, glory, splendour, wisdom, and dispassion.
The purport of the letter va is that elemental spirit in which all
beings exist, and which exists in all beings. And thus this great
word Bhagavan is the name of Vāsudeva, who is one with the supreme
Brahma, and of no one else. This word therefore, which is the general
denomination of an adorable object, is not used in reference to the
supreme in a general, but a special signification. When applied to any
other (thing or person) it is used in its customary or general import.
In the latter case it may purport one who knows the origin and end and
revolutions of beings, and what is wisdom, what ignorance. In the
former it denotes wisdom, energy, power, dominion, might, glory,
without end, and without defect.
Hari Vamsa states:
vede rAmAyaNe punye bhArate bharatarShabha | Adau chAnte cha madhye
cha hariH sarvatra gIyate ||3-132-95
In Vedas, in the Ramayana and in the sacred Bharata, O chief of
Bharata’s race, Hari is sung everywhere, in the beginning, the middle
and at the end.
Thus, Krishna says in Bhagavad Gita(15.15)
vedaiś ca sarvair aham eva vedyo vedānta-kṛd veda-vid eva cāham
I am that which is to be known in all the Vedas. I, indeed, the knower
of the Vedas and the author of the Vedanta.
Similarly in the Bhagavata Purana(11.21.42-43), Krishna speaks to Uddhava:
kiṁ vidhatte kim ācaṣṭe kim anūdya vikalpayet ity asyā hṛdayaṁ loke
nānyo mad veda kaścana māṁ vidhatte ’bhidhatte māṁ vikalpyāpohyate
None except me knows what is really taught by the commands and
prohibitions as laid down in the Karmakanda ; what is really expressed
by the Mantras in the Devata-kanda, or what is the purpose of the
passages to be found in the Jnanakanda. All the Karmakandas refer to
me because I am the great sacrificer ; all the Mantras praise me
because I am the highest Devata ; and all the Jnanakanda refers to me
because I am the creator of the world and withdraw it again to myself.
Verily, I am this all. Again, Scriptures enjoin duties as my worship,
use Indra and all other names as my appellation, the texts that
prescribe, as well as prohibit acts, point to me; so, in such a state
none other than myself understand their true meaning.
You may also read commentary on the below sutra:
tat tu samanvyat
(But Vishnu is the subject matter of all the Vedas) because such is
the appropriate interpretation of all the texts.