Iron Man Returns
It's rare to say that a sequel is better than the original, but Iron Man 2 improves on the original in just about every way.
The plot is superb. We know who Tony Stark is and here we get to see a more vulnerable side to him, as we learn early on that something is wrong with him. The main villain in this film is much more menacing and believable, and is played brilliantly by Mickey Rourke. Another villain is Sam Rockwell playing Justin Hammer, a twisted version of Tony Stark. There's also Scarlett Johansson as the quirky yet likable Natalie Rushman (who has a secret of her own).
My only complaint is the film feels rushed at times, and that major problems were solved too easily. Also, the humor got a bit too silly sometimes.
If you're avoiding this because they replaced Terrence Howard with Don Cheadle, don't. Cheadle nearly steals the entire movie; he's that good. And then of course there's Gwyneth, who has a much more prominent role. Altogether, it's a great film and a great cast.