The second Iron Man movie but it doesn't hinder Paramount from planning a third Iron Man movie. No need to be a soothsayer to foresee that IM2 will a huge success and that the public will be craving for yet another Iron Man movie. So it does make sense to plan Iron Man 3. And it was actually confirmed by Director Jon Favreau himself:
IM2 can of weave its story around the demon in the bottle storyline, with Tony Stark, aka Iron Man, facing the aftermaths of his superhero coming out. So if our hero can overcome his personal problems and the villains from IM2, we can expect his real nemesis to show up in the following movie, namely the evil Mandarin, in Iron Man 3.
Actor Faran Tahir, who played Raza, the mysterious leader of the 10 Rings in the first Iron Man, gave some hints that the Mandarin could indeed be the next villain pitted against Iron Man:
He has known for a long time where he wants to take it and I don't want to speak for him, but my feeling is that the second movie didn't want to just follow a singular storyline. They wanted to expand the characters to show that it's not a linear saga like Lord of the Rings. There's a linkage somewhere in there, but the big idea is to do at least one more Iron Man movie and then go into The Avengers."
Hulk could be an other option, I think he will rather show up as the villain in the Avengers. So I can see only the Mandarin as a credible villain in the movie Iron Man 3.
Now let's think about a possible release date for Iron Man 3. Thor will hit theaters in May 2011 and The Avengers movie is set for a release in Summer 2012. So unless they rush the development of Iron Man 3 to release it in between, the movie Iron Man 3 is more likely to be in 2013 or later.
But releasing Iron Man 3 after the Avengers would bring some complexity because of the new continuity. If they want to keep it simple it would be better to release Iron Man 3 between Thor and the Avengers....
Who would be the best villain for Iron Man 3 according to you? And Should the movie be released before of after the Avengers?