For my money, "The Dark Knight" is easily the best superhero film till date.
It's only too easy to come out of "The Dark Knight" and wonder if the film should really have been titled "The Joker". But that is stating the obvious. Drama is conflict and an extraordinary conflict requires an extraordinary antagonist. All great "action" films have memorable villains. It's the rule.
One of the reviewers got it right - I believe it is Ebert - when he called Batman merely one of the ensemble players, not the lead in the traditional sense. When reading the comic-books, it was easier to appreciate this fact: those books were as much about the villains as they were about Batman. In fact, the decision to buy a Batman comic-book was often dependent on which villain got the "top billing" on the front cover.
I disagree with Falstaff when he describes Nicholson's Joker as "a bizarre, larger-than-life arch-fiend, who seemed to have stepped straight out of an animation. He was psychotic and scary, but in a way that conformed to the boundaries of the genre."
IMO, Nicholson tried too hard to be scary-funny and was neither scary nor funny. It took me completely out of story-universe. But Ledger's character elicited laughter, sympathy (when he first starts talking about those scars), fear and revulsion. This is what the Joker's character is all about. Here's a sick, crazy villain doing all those crazy things and what you want most, at that point in the story, is for Batman to show up and straighten up things. Totally unlike Burton's Batman, in which I just wanted to watch Nicholson do his shtick.
What Christopher Nolan's film manages to convey at the end is a (temporary) sense of security: yes, villains like the Joker are out there, but Gotham has Batman. He may be the Hamlet of superheros, but at the end of this adventure, Batman's voice has certainty and confidence. The Joker may have had the most interesting lines in the film, but it is Batman's conviction about his actions that will bring me back to the next episode.
P.S.: For the first time on screen, Batman has that glowing, "white-eyed" look. Need I say more why I think this is the best Batman film ever?