My vote would have been for Thanos for obvious reasons, but since he's not on the list, I have to go with Galactus:
Galactus was instrumental in the defeat of the Annihilation Wave and defend the Universe on more than once occasion. He defeated Tyrant when even Thanos, Silver Surfer, Jack of Hearts, Gladiator and other power houses were unable to.
Galactus destroyed the Ultimate Universe and subdued "Gah Lak Tus" who was virtually unstoppable in its respective universe.
Galactus has been one of the few entities to survive direction assaults from the Phoenix Force for the longest period of time.
Galactus was able to infiltrate the Realm of Aggomoto to retrieve Dr. Strange during the Infinity War.
He is supposed to be a Universe-Level Powerhouse, but his influence extends beyond one universe. In proportion to his place in the cosmic hierarchy he has been most impressive in my opinion.