If he's gonna get flamed out the wazoo for saying that, let's see what people think of this:
Heath Ledger is a better actor than Jack Nicholson.
There. I said it. That's my honest belief, and I think that when both of their careers are over, history were shine in Ledger's favor. Sure, comparing the two in such a way may seem like heresy now, since Jack is so old and respected, and we've only seen a very small portion of Ledger's serious acting, but let's look at it critically:
Everything Jack does is distinctly Jack. He does well with his roles, but every role he does is basically a variation on himself. He is incapable of seperating himself from the character. When you see Nicholson perform, you basically know what you're getting.
Ledger? Ledger becomes the character. His performance in Brokeback Mountain was utterly convincing (and heartbreaking), and there was nothing about his performance (short of the fact that you could look at his face and know it was him) to suggest that it was Heath Ledger. In that movie, he is the character. From the looks of it, it'll be the same way with I'm Not There and The Dark Knight. All three of those performances will be totally distinct; completely different characters, different people. And he's still young. He's only 28. It's only going to get better from here.