Saitama would still win, and not even thanks to his power, but thanks to his omnipotent rule of funny.

Thanos: *snaps*

Saitama, quite literally:

Stop, Saitama is not a character you can compare power level wise. He is fucking designed to be busted. He is like if Bugs bunny appeared in a super hero comic. He'd break every rule and law there is because they don't apply to him. That doesn't mean Saitama is the strongest character ever made, it means he doesn't even play the game.

This is like if you were playing DND and someone decided to play as the monopoly hat. Then people having serious discussions about what the limits are of the monopoly hat within the DND universe.