The Avengers by a hair, I think. Iron Man is incredible but something about watching all those characters speak Joss Whedon dialogue at one another puts it over the top. Plus the pure spectacle of seeing them all on screen together for the first time is hard to beat.
The Avengers edges the first Captain America by a hair for me. I really love the style that Joe Johnston brought to it.
Captain America for me. Love it so much. Style was so different from the other superhero films. Iron Man is a close 2nd.