It has to be The Dark Knight. I've always been a Batman fan, and I was looking forward to this movie so much, but it exceeded my highest expectations.
Reasons it is awesome:
1. More than any other Batman film, it addresses why there must be a Batman. It shows why pretenders can't do what Batman does. It deals with the limits of the Law and the compromises that people make when they want to feel safe, and shows a man who is willing and able to defy both of them in order to do what is necessary to defeat true evil.
2. It is a brilliant, layered film. It transcends the superhero genre. You could list it among great crime dramas and I don't think it would feel out of place. And there is so much to the intelligent script. (I love how the Joker completely changes his plans after he comes face to face with Batman.) Someone in the media (forget who) called it the most important post 9/11 film to have come out. That's probably a loaded label, and debatable, but it does describe how the film tries to tackle huge questions of personal liberty vs. security in a world where terrorism can touch anyone, and it deals with collectivism vs. individualism (Watchmen does this also). Is Batman a hero in the end because he defeats the Joker or is he a bad guy because he tramples the civil liberties of the entire city to find him?
3. It brings Batman into the real world. Gotham just looks like Chicago. The Joker is just a really smart, cruel man in face paint. I'm a big fan of the dark, stylized world of some of the other movies (Rocket penguins? I'm there!), including Batman Begins, but a real world with real danger worked here. A madman with a bomb is scarier than a device that magically evaporates all the water in the area (except what's in the human body). And a truly frightening scenario makes for a more heroic victory.
4. The Joker. I love Jack Nicholson's Joker, and I had serious reservations about Heath Ledger and the realistic, scarred Joker of the Nolanverse. Until I saw the first trailer. And then the film. Wow. You don't even see Heath Ledger in there. His best performance ever, and a great take on a classic bad guy.
5. Three great dramatic face-offs. My favorite scenes in the film are: Batman vs. The Joker in the interrogation, The Joker trying to convert Harvey Dent to his worldview in the hospital (love the nurse outfit), and Batman, Gordon, and Two-Face facing off at the end of the film (Explosives and high tech car chases are set aside for a chilling dramatic climax: just three guys, a gun, and an innocent child...).
There's more, but that's the heart of it. It's just great to watch a movie with your favorite superhero, written by people who get the character and want to give you more than just a Summer popcorn flick. (And David S. Goyer has written the upcoming Superman film, so I live in hope for my second favorite superhero...)
Runners up:
Superman: The Movie (When you're a kid, this film is great. Makes you want to tie a towel around your neck and jump off furniture.)
Kick-Ass (Hit Girl is the coolest action heroine since The Bride in Kill Bill, and the movie itself is intense, violent fun.)
Blade (Frost is a great villain. Also written by David S. Goyer.)
The Avengers (I can't believe this movie is that good. Surpasses all my hopes for it. Perfect example of the whole being greater than the sum of its parts.)