wasnt that the point of ledger's joker? to be detached from human emotions? to not have heart? nicholson's joker started off as your average criminal and then had the transformation. heath's joker is a mystery, and that makes him a much more terrifying villian than nicholson's joker. i dont see the point in bringing mimicry into the conversation, but in ledger's defence take the first scene where he's describing how he got the scares and how impassioned he gets, or in his final scene where neither of the boats detonate and you see true dissapointment on his face. if a good mimicry job can nail those moments then i'm sure it's possible to get to the heart of nicholson's performance as well. what makes ledger's performance number 1 for me is simply the fact that it had the strongest impact on me which no doubt has a lot to do with the fact that, as stefen said, ledger is virtually unrecognisable in the role.
Again, you don't just need the evil doers to be sympathetic or have heart. What I'm asking for is a roundness of character that isn't found in him at all. The writing didn't allow much room for Nicholson, but his venerable skills as an actor gave the character some girth. There were dimensions to his madness. Yes, it helped that his character had a chance to transition from criminal to psycho, but the way Nicholson commands even the pyschosis itself has a lot of flexibility to it. I call up Anthony Hopkins original performance as Hannibal Lector to show example. The character is meant to become sympathetic because of his relationship to Starling, but exclude her and you see a character that is solely evil but has different dimensions to him because of Hopkins skills.
Ledger does a fine job with Joker, but he puts so much emphasis on the easy scare factors of his jokers that the antics, gestures and voice are the most memorable thing about the character. The heavy handed focus on these characteristics keep the performance out of greatness category. The performance is wonderful in its own way, but all I'm saying is that Ledger played the performance to such an artificial monster-like quality that he also made it easy for other actors to copy, which isn't a good commodation. His performance is different than Hopkins so I understand the idea they are incomparable, but I think that Ledger's performance is below Hopkins for reasons already stated.
I also didn't see true dissapointment in Ledger's face with the foiled bombing. I saw a quick shot of confusion close up on the face, but then it was immediately to frustation because the Joker immediately did a violent act to Batman right after. There the camera was pointed at him from the side and at an odd angle. It wasn't in the best place for revelatory expressions from Joker. I think the camera was in the best place to gage Joker's reaction on a general level, but mainly it was a good stationery place for the camera to cut to the action immediately coming after it. But even if you were right I think that one point isn't much evidence anyways.