"Why so serious?"
"Do you wanna see a magic trick?"
etc, etc, etc.
The best of them all, though, is a scene in which he does practically no talking at all...
In Which We Meet the Antagonist
There are very few scenes that introduce the movie viewer to the antagonist in such a excellent way. From the jarring, noisy music to the batshit insane, yet flawless plan (which, in setting the stage for the rest of the movie depends on any number of random things to go perfectly right - they all do, of course), the bank heist is a great introduction.
The scene was provided online as a sort of teaser trailer for The Dark Knight. I saw it once, then again, then again, then spent the next months eagerly anticipating the movie. It'd be hard for any movie to live up to the expectations set by this scene, but long ending aside, I'd say TDK came close.