The Celestials are one of the oldest entities of the Marvel Universe who are literally the space gods. They wield the power cosmic and their abilities are limitless. They have a tendency to destroy the planets and beings like Odin and Phoenix Force have to go all in just to stop one of them down. They were introduced for the first time in the MCU in the Guardians of the Galaxy but it failed to realize the potential of these cosmic creatures.
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.