Iron Man > Iron Man 3 > Iron Man 2
One of the best superhero origin movies, without a doubt. I think the story in this one was the best as well, simply because Iron Man's origin and character growth as he becomes a hero is one of the best of any superhero. Add all of that along with an unforgettable performance from Robert Downey Jr., you get lightning in a bottle.
Iron Man 3:
Good story, fantastic action, funny, quotable ... Just an all around great comic book movie, action movie, and Christmas movie.
Iron Man 2:
This movie may be the "worst" of the three, but I still think it's a lot of fun. I don't think it deserves the hate that it gets; not by a long shot. A lot of the stuff covered in this movie (Nick Fury, Black Widow, etc.) was necessary as a stepping stone for The Avengers.
Easily my favorite superhero movie trilogy to date, as I've enjoyed all three movies quite a lot.