TDK was a great movie I think, I've only seen a bootleg copy of it on my PC so far, so I'm looking forward to seeing it again. I think Heath Ledger was great as the Joker, just his mannerisms where great, he really made you feel like this guy is criminally insane. I also liked how there was a couple of times you thought it was over, but it wasn't.
Too bad Heath Ledger died though, because you just know that they would have brought him back in a future sequel, maybe not the next one, but a later one, and I'm sure it would have made for again a great movie.