two words.
THE BEYONDER!! (Secret wars II, for those of you yungins)
and about the only thing that could beat the beyonder would be the Living Tribunal. I'll quote the living tribunal when adam warlock had the infinity gauntlet and was on cosmic trial and he decided he wanted to keep the gauntlet and tried to destroy all the cosmic entities attending the "trial", and the Living Tribunal just snapped his fingers and it was as if nothing had happened.. and he said....
"I RePrEsEnT PoWeRs ThAt DwArF eVeN YoUr MiGhT"
So there you have it.. in the marvel universe, the Living Tribunal is the end all be all.
(but if we're talking about just the earth, then the HULK!!!!!)
(on second thought, if it gets brownie points, then i think Cyclops would win
)