The Dark Knight
I think this is the best of the Batman movies when it comes to really probing the moral ambiguities surrounding Batman's vigilantism, but also human nature in general. When was the last time you saw a bad guy conduct a "social experiment" in real time?
Heath Ledger was great as The Joker. Personally, I don't see anything Oscar-worthy about the performance... he does a convincing crazy guy, but I don't think that takes as much skill as what he did in Brokeback Mountain or Monster's Ball.
Christian Bale was also great as Bruce Wayne, and as Batman whenever he wasn't talking. I do still prefer Michael Keaton's batman to all others. Which is something I never thought I'd say.