Starting with Brahma Samhita which is written by Lord BRAHMA himself who everyone says is the supreme lord along with Lord Shiv and Lord Vishnu. In the whole of Brahma Samhita, there are numerous shloks where he has stated that Lord Sri Krishna is supreme and that he has is also the cause of the generation of Lord Vishnu and not only lord Vishnu but the generation of MAHA-Vishnu as well.
Below, I am stating 3 shloks that sum up those numerous shloks.
5th adhaya, 39th shlok:
Ramadimurtishu kalaniyamena tishthan
Nanaavtarmakrod bhuvneshu kintu,
krishnah swayam sambhavatparamaha puman yo
Govindmadipurasham tamaham bhajami
Meaning:-- Jinhone SHRI RAM, Narsingh, vaman ityadi vegrahon mein niyat sankhya ki kala roop se stith reh kar jagat mein vibhin avtaar liye, parantu jo BHAGVAN SRI KRISHNA ke roop me SWAYAM PRAKAT huye, un adi purash bhagvan Govind ka main bhajan karta hoon.
5th adhaya, 43rd shlok:
Goloknamni nijdhamni tale cha tasya
Devimaheshharidhamsu teshu teshu
Te Te prabhavnichaya vihitascha yen
Govindmadipurasham tamaham bhajami
Meaning:-- Jinhone golok namak apne SARVAPARI(the uppermost) dham mein rehte hue uske niche stith kramsha Vaikunt lok jahan pe Vishnu bhagvan virajte hain, Shiv lok jahan Shiv bhagvan virajte hain, Devi lok namak vibhin dhamon(other dhams) ke vibhin swamiyon(the onwners) ko yathayogya adhikar pradhan kiya hai, un adipurash(meaning the first man) GOVIND ka main bhajan karta hoon.
5th adhaya, 48 shlok:
Jeevanti lomvilja jagdandanatha:
VishnurMahan sa eha yasya kalavishesho
Govindmadipurasham tamaham bhajami
Meaning:-- Maha Vishnu ke rom chidron se prakat Brahma, avem bahutik brahmandon ke anya swami gadn unke(maha vishnu ke) ek nishwas jitne kaal tak hi jivit rehte hain, paruntu jinki ek vishisht kala matra(i.e. an extraodinary art of Lord Krishna) Maha-vishnu bhi hain, un adi purash GOVIND ka main bhajan karta hoon.
SKAND PURAN MAHATMA
These 2 shlok should be enough from Skand Puran since it says these two shloks alone say it ALL so I am not writing the further shloks.
3rd adhaya, 21 and 22 shlok
Ikshanchakre yada Krishno mayapurasharupdrakh
Brahma Vishnu Shivaschapi rajah satvatamogudaya|| 21
Purshastraya utasthuradikaranstadadishatautpatau Palane Cheva Sanghare Prakramedra Tana|| 22
Meaning:-- Apni maya se purash roop dharan karne vale Bhagvan Sri Krishna ne jab shrishti ke liye sankalp kiya arthat shrishti ko utpan karna chaha tab unke divya vegrah se(meaning sharir se) teen purash prakat hue. In mein rajo gudan ki pradhanta se Brahma, satva gudan ki pradhanta se VISHNU aur Tamon gudan ki pradhanta se Rurdra(Bhagvan Shiv) prakat hue.
Bhagvan Sri Krishna ne in tino ko kramshah(respectively), jagat ki utpati, palan aur sanghar karne ka adhikar pradan kiya.
BRAHMA VAIVART PURAN
Brahma Khand, 1st shlok:
Surasacha sarve manvo munindraha
Namnti devyah pradramaami tamvibhum
Meaning:-- Ganesh, Brahma, Mahadev ji, Devraj Indra, ShehshNag adi sab devta, Manu, Munindra, Saraswati, Laxmi tatha Parvati adi deviyan bhi jinhe mastak jhukati hain, un sarva vyapi parmatma ko(Shri krishna) main pranam karta hoon.
Brahma Khand, 4th shlok:
Vande Krishnam Gudatitam param Brahmachutam yatah
Meaning:-- Jinse Prakriti, Brahma, Vishnu tatha Shiv adi ka aavir bhav(have generated) hua hai, un Trigunatit parabrahm parmatama achut Shri Krishna ki main(Rishi Sutji) vandana karta hoon.
The THIRD ADHAYA further states that:--
Sri Krishna ke dwara srishti ka arambh tatha Narayan, Mahadev, Brahma, Dharm, Saraswati, Mahalaxmi aur Prakriti(Durga) ka pradhurbhav bataya hai.
So, one can read it in depth from there if they want to know more.
Saa Sangyam yati Bhagvaneka av Janardanha:
Meaning:-- Vaha ek hi Bhagvan Shri Krishna(Janardhan) jagat ki shrishti(generate), stithi(preserve) aur singhar(destroy) ke liye Bhagvan Brahma, Vishnu aur Shiv - in teen sangyaon(roop) ko dharan karte hain.
Twamev jagato nabhi raradnamiva sanshrayaha
Karta paharta pata cha trayalokyam twum trayemayaha
Balram ji is saying to Lord Shri Krishna:--
Meaning:-- Pahiyon ki nabhi jis prakar aron ka ashraya hoti hai usi prakar aap(bhagvan sri krishna) hi jagat ke ashraya, karta(generator), harta(destroyer) aur rakshak(preserver) hain tatha aap hi trayalok ke swarup aur veda trayi maya hain.
In the Padma Purana it is stated, by scrutinizingly reviewing all the revealed scriptures and judging them again and again, it is now concluded that Lord SHRI KRISHNA is the supreme absolute truth, and thus he alone should be worshipped.
Padma Puran, Uttar Khand, 1st adhyay, 1st shlok
TapatryaVinashaya ShriKrishanaya Vayam Namah
Meaning-- Bhagvan ke teen swarup hain...sat, chit and anand. Vo bhagvan vishwa(earth) ki utpati(generate) karte hain, uska palan(preserves) karte hain tatha samaye aane par, sanghar(destroy) kar dete hain. Jo teen tarah ke tap ko nasht(destroy) karne vale hain...AISE SHRI KRISHNA BHAGVAN KO HUM NAMAH KARTE HAIN.
According to Garg Samhita also, Lord Sri Krishna is supreme. Of which shloks, I have already provided in the MB Forum.
Vasudeva Para Veda Vasudeva Para Makha
Vasudeva Para Yoga Vasudeva Para Kriya --- 1.3.28
Vasudeva Param Gyanam Vasudeva Param Tapah
Vasudeva Paro Dharmo Vasudeva Para Gati --- 1.3.29
Meaning-- Veda'on'(rig veda, yajur veda etcc etcc) ke tatparya SHRI KRISHNA main hi hain. Yagyon ka udeshya(AIM) SHRI KRISHNA hi hain. Yog(dhyan) SHRI KRISHNA ke liye hi kiye jaate hain. Aur samast karmon ki pari samapti SHRI KRISHNA mein hi hain. Gyan se Brahma swarup SHRI KRISHNA ki hi prapti hoti hain. Tapasya SHRI KRISHNA ki prasanta(happiness) ke liye hi ki jati hain. SHRI KRISHNA ke liye dharmon ka anushthan(Puja etc) hota hai. Aur sab gatiya(living and non-living things) SHRI KRISHNA mein hi sama jati hain.
English Mean -- After Describing various incarnations of the Lord such as Rama, Balrama, Vamana, Nrsimha, and Vishnu, Sukadeva Goswami states:
"All of the above mentioned incarnations are either plenary portions of portions of the plenary portions of the Lord, but Lord Krishna is the original Personality of Godhead" --- Srimad Bhagavad 1.3.28
IMP...SRIMAD BHAGVAD - PRATHAM SKAND, 3rd ADHAYA, FROM 30TH SHLOK TO 34TH SHLOK, its been told that SHRI KRISHNA is the SUPREME PERSONALITY OF GOD(SARVAPRI). He generates, preserves and destroys the world when the time comes.
34TH SHLOK SAYS:--
BhavaYatiyash Satuen Lokan Vai Lok Bhavana
Meaning:-- Bhagvan SHRI KRISHNA hi sampurna(ALL) lokon(14 loks) ki rachna karte hain aur devta, pashu-pakshi, manushaya adi yoniyon mein leela avtar grehan karke...satva guran ke dwara jeevan ka palan poshan karte hain.
11th SKAND, 5th ADHAYA, 33rd SHLOK
Dheyam Sada ParibhavganaMabhishtaDoham
TirthaSpadam ShivViranchiNutam Sharanyam
Brahtyartihum Pranatapal BhavabdhiPotam
Vande Mahapurash Te CharnarVindam
Meaning:-- Prabhu(SHRI KRISHNA) aap sharnagat rakshak hain(one who protects those who surrender to him), aapke charnar vinda sada sarvada dhyan karne yogaya, maya moh ke karan hone vale sansarik parjayeyon ka anta kar dene vale tatha bhakton ki samasth abhishta vastuon ka daan karne vale kam dhenu swarup hain. Vetirthon ko bhi tirath bana dene vale swayam PARM TIRTH SWARUP HAIN. SHIV, BRAHMA, ADI BADE BADE DEVTA UNHE NAMASHKAR KARTE HAIN.
Ekam Shastram DEVAKIputragitameko devo devakiputra aev
Eko Mantrastasya Namani Yani Karmapiyekam Tasya Devasya Seva
Meaning:-- Lord Sri Krishna is supreme and is the true meaning of puran, shastra etc. and everything is generated from him, be it lords, humans or any other thing.
SHLOKS 11.10, 11.11, 11.12 and 11.13 -- All these are talking about the VIRAAT swarup of Lord Sri Krishna.
11.10 and 11.11 mentions that Arjuna saw in that universal form unlimited mouths, unlimited eye, unlimited wonderful visions. The form was decorated with many celestial ornaments and bore many diving upraised weapons. He wore celestial garlands and garments, and many divine scents were smeared over his body. All was wondrous, brilliant, unlimited, all expanding.
11.12 - Divi suryasehstrasaya bhavedyugapaduthita
yadi bha:sadrashi saa syabhadasastsya mahatmanaha
Meaning:-- If hundreds of thousands of suns were to rise at once into the sky, their radiance might resemble the effulgence of the supreme person Lord Sri Krishna in that universal form.
11.13 - Tatryiakastham jagatKRISHNAM pravibhaktamanekadha
Apshayadevadevasaya Sharire Pandavastada
Meaning:-- Pandu putra Arjun ne us samay anek prakar se vibhakt arthat alag alag sampuran jagat ko DEVON ke DEVA Bhagvan SRI KRISHNA ke ush sharir mein stith dekha artath sampuran jagat ko, Arjun ne bhagvan mein dekha.