Board Thread:General Discussion/@comment-6527064-20150807232945/@comment-34326521-20150807234742

Doctor Manhattan - an omnipotent, nigh-omniscient, immortal, indestructible hero who literally created himself (look up "causal loop"). He can wipe Ben's entire timeline from existence just as easily as he can disintegrate the Omnitrix.

Hulk - Hulk gets stronger the angrier he becomes. Unless Ben finds a way to knock him out instantly (Echo Echo or his Ultimate would work), or if Alien X is feeling cooperative, Ben's toast.

Wolverine - immortal, indestructible. Definitely a challenge, but I doubt he can stand up to fire-based aliens.