With the world now aware of his dual life as the armored superhero Iron Man, billionaire inventor Tony Stark faces pressure from the government, the press, and the public to share his technology with the military. Unwilling to let go of his invention, Stark, along with Pepper Potts, and James "Rhodey" Rhodes at his side, must forge new alliances - and confront powerful enemies.

Dr. Bruce Banner, thanks to a gamma ray experiment gone wrong, transforms into a giant green-skinned hulk whenever his pulse rate gets too high. Meanwhile, a soldier uses the same technology to become an evil version of the original.

I can't really single one out, but out of all the upcoming Marvel movies, I may be looking forward to Thor the most.
Don't get me wrong, I want The Avengers movie and the Captain America: The First Avenger movies both to be awesome, but Thor, I think will be trickier to pull off.