Since everyone is doing it,
1) Iron Man
2) The Incredible Hulk
3) Iron Man 2 (not as good as the original, but in some ways I'd rather watch it if given the choice between the two. Love all the element creation stuff, Sam Rockwell, the race track sequence, the final confrontation, plus the Walt Disney style Stark Expo. On the other hand, I HATE the party sequence. If they had done that scene differently, the entire film would have been remembered a lot more fondly.)
4) TIE: Thor & Captain America. Thor is a more visually interesting film (except for the town stuff), and they captured Loki perfectly. On the other hand, Captain America is a well-executed love letter to 80's Spielberg, plus a decent throwback to the old serial type adventures. I think it's Joe Johnston's best film, as well. It's also a better film than Thor structurally speaking (may actually be the best structured of the entire lot). At the same time, because it feels so familiar and nostalgic, I don't really think the material itself manages to ascend on its own merits. Bottom line: I don't think either are particularly great (although I enjoy them both), and each excels in its own area. I also typically associate them with one another thanks to their release dates being so close together. Finally, considering everyone expected them to be horribad, I think Marvel did very well with both of them.
Honestly, the only really great film they've done is Iron Man, and even then, I've always found that it has some unfortunate third act issues. The Incredible Hulk was very interesting too. At the same time, I actually feel like if they had retained a couple of the more philosophical elements of Ang Lee's film, it could've had a shot at being one of the best films in the entire genre. Don't get me wrong though, nearly every element of TIH is an improvement over HULK.
Cap and Thor are fun adventures that achieve the dual task of building the Avengers roster, while still being decent and entertaining in their own right. Marvel did well by maintaining the creative integrity of the people who worked on these films, while still making the adaptations feel like actual filmic representations of the comic books they're based on. The degree to which they've captured the way those comic books actually feel (in my limited experience), is what makes this collection of films unique. Still, without The Avengers coming out this summer, I'm not quite sure they'd feel entirely relevant in the way films like Iron Man and Spider-Man do; that's probably not a fair criticism.