Although my favorite parts of the film are pretty divided, I have to say one of those moments is the final Joker scene. After he says the "madness is like gravity" line, that laugh... it's monstrous, and more frightening (IMO) than anything else he did in the entire film. And that's just from listening to it, but it goes even further when you put it in the proper context. As far as he's concerned, he won. Whenever he starts to laugh for the final time, it's like he's saying, "I just fucked over Gotham, and there's nothing you can do about it."
The Orson Welles shot (low angle shot) in which The Joker is looking at fallen from Batpod Batman with Gotham buildings as background is my favorite shot from all the movies in 2008.