Vic Tional at 2012-07-21 17:52:46:
Like Scott says, there are a lot to choose from. Happy to get the ball rolling:
The Joker: Oh, you. You just couldn't let me go, could you? This is what happens when an unstoppable force meets an immovable object. You truly are incorruptible, aren't you? You won't kill me out of some misplaced sense of self-righteousness. And I won't kill you because you're just too much fun. I think you and I are destined to do this forever.
Batman: You'll be in a padded cell forever.
The Joker: Maybe we can share one. You know, they'll be doubling up, the rate this city's inhabitants are losing their minds.
Batman: This city just showed you that it's full of people ready to believe in good.
The Joker: Until their spirit breaks completely. Until they get a good look at the real Harvey Dent, and all the heroic things he's done. You didn't think I'd risk losing the battle for Gotham's soul in a fistfight with you? No. You need an ace in the hole. Mine's Harvey.
Batman: What did you do?
The Joker: I took Gotham's white knight and I brought him down to our level. It wasn't hard. You see, madness, as you know, is like gravity. All it takes is a little push!
http://movieclips.com/Z6xq-the-dark-knight-movie-the-battle-for-gotham/
As far as screenwriting takeaway, it sums up the relationship between The Joker and Batman beautifully, it's funny, it's memorable. They're not that different, the two of them - 'unstoppable force meets an immovable object', which one's which?
The reversal where The Joker makes his revelation about Harvey Dent to get the last laugh, pulling the rug from under everyone, is fantastic. Just a perfect closing moment for a great character.
On a separate note:
Alfred Borden: You went half way around the world... you spent a fortune... you did terrible things... really terrible things Robert, and all for nothing.
Robert Angier: For nothing?
Alfred Borden: Yeah
Robert Angier: You never understood, why we did this. The audience knows the truth: the world is simple. It's miserable, solid all the way through. But if you could fool them, even for a second, then you can make them wonder, and then you... then you got to see something really special... you really don't know?... it was... it was the look on their faces...
http://www.youtube.com/watch?v=P5eM5ww-9N4
(speech from about 3:40)
Again, its moving and memorable, but it's more than that. Angier was never Borden's equal as a magician, nor as a human being, yet he understood something about magic and humanity that Borden missed. I love that speech.