Real Name: Thanos

First Appearance: Iron Man #55 (Feb. 1973)

Powers: You name it. Super strength, the power to project and absorb massive amounts of cosmic energy as well as a good deal of durability.  Moreover, his super-intellect makes his knowledge of alien technology incredibly dangerous.

Hero or Villain: Villain

Why: One of the most powerful beings in any corner of the Marvel Universe, the alien Thanos has made the eradication of all sentient life his goal.  Moreover, you remember the Infinity Gauntlet that was displayed among Odin's trophies in "Thor"?  That's one of the most dangerous weapons in the Marvel Universe (think of the ring from "The Lord of the Rings" but much worse) and a perfect way to bring the character to the big screen.  The problem is, of course, he's too powerful to be the adversary in just a "Thor" sequel. Only the combined power of the Mighty Avengers can stop him (at least in our opinion).