Mr. Immortal - His power is that he will be the last mutant alive at the end of time. What's even funnier is he is suicidal.
Nick Fury - Possibly the most powerful regular human alive. Granted its freakin Thor, if anyone has a way to beat him, its Fury.
Absorbing Man - Creel is a threat to the Avengers as a team. I think duck is an appetizer for him.
Punisher - Considering Jean Grey is one of those characters whose powers is to die over and over again (also see Mr. Immortal and Elektra), and she isn't immune to a shotgun blast to the face, Frank has this one in the bag.
X-men - Thing goes head to head with Colossus and Rockslide; Human Torch with Sunfire and Magma; Invisible Woman with White Queen; Mr. Fantastic with Mercury. Oh wait, that still leaves Wolverine, Cyclops, Angel, Beast, Storm, Rogue, and about 20 more characters. No contest.
*On a side note, if you need another team, why not throw Power Pack into the mix for SnG?