The Chaos King
Thanos may be powerful but he ain’t nothing in front of this guy. Amatsu-Mikaboshi, aka, The Chaos King is the embodiment of darkness who existed even before the creation. He is more of a primal force than a physical body and Gods like Odin and Zeus are powerless in front of him. During the Chaos War, he enslaved everyone, defeated Galactus, destroyed Hell and even scared the shit out of Death.