Brick
Took a while before it started gaining momentum, but once it did, I was totally engulfed in the main character's persistence.
Having seen Manic, Mysterious Skin, and now Brick, I am convinced Joseph Gordon-Levitt not only has the looks, but also the talent to play the next Joker should Nolan decide to use him again. And even if he doesn't, this still is a talented young actor with an eye for good scripts.