Well, I haven't seen Ledger's performance yet, so I really can't say without having seen it, I suppose. However, I'll give it a shot.
I don't think I will like Ledger's performance of The Joker all that much, because it sounds too dark to me, but I may change my mind once I have seen it.
I love Nichelson - he's a riot, and he made The Joker famous again.
However, on this one, I'm going with the original - Ceasear Romero (TV's Joker on Batman show). He was one class act - and extremely hard to replace, immitate, in my opinion.