The Avengers
On top of being an awesome, extremely fun action movie, the characters are all portrayed very well and the way they come together feels natural and not at all convoluted.
Iron Man
A great adaptation of the character and an all-around great character arc. RDJ was the perfect casting choice too.
Superman: The Movie
Visually, it's a technological breakthrough that still holds up. For the story and characters, Donner completely nailed the core of Superman, and his decision to cast an unknown in order to put the audience in the mindset is a model for future filmmakers (unless there's a famous actor who's just an irresistible choice like Downey).
Spider-Man 2
Marvel editor Steven Wacker said the best Spider-Man stories are the ones that are fun and take Spidey to emotional depths. This is one of those stories.
X-Men: First Class
The story of Xavier and Magneto is one of the best in the Marvel movies and I loved the 1960s spy feel of the film.
Sin City
Beyond the coolness of making such a direct screen translation, Sin City shows a really interesting take on the noir genre.
American Splendor
I love the narrative style of having the real Harvey Pekar and Paul Giamatti playing the younger Pekar. It really strengthens his development.
Road to Perdition
Not particularly original, but very well made in all ways, from suspense to acting to the beautiful visuals.
Scott Pilgrim vs. the World
Edgar Wright had a great handle on how to change the comics to work better on screen while still maintaining the spirit and hilarity of the source material.