I'm quite a fan of Heath Ledger's - and I found myself completely forgetting that it was him under that ghoulish makeup. His performance is so incredible and so chilling. He didn't get an Oscar for BM, but he ought to for this. His Joker is what made me decide that I actually prefer the ultra-realistic, super gritty Batman movies (this and Batman Begins) to the previously preferred camp classics. This Joker is better than anything I've ever seen in a superhero movie, I have to say, and one of the best adaptations.
Oh and Christian Bale is amazing too. And Aaron Eckhardt.
The only sad thing is that we won't get to have a Joker in future movies - honestly, who would want to try to follow that performance?
Seen it three times and I'd see it again in a heartbeat. Yeah, even at 8.50 a ticket.