Iron Man is among the best ever of the superhero movies. Great acting from a great cast, a good story, and it'll put a smile on your face because it doesn't take itself too seriously (but it is definitely not a "light" movie). In the vein of X-Men, and Spider Man, there's just about a perfect balance between comic book, human drama, humor, and compelling story line. (And actually the caliber of actors and "near-believability" of the story here probably nudges this one ahead of those other two in my opinion.)
One of the few times I've left the theater wishing the sequel was coming out next week.