Personally, I consider Batman Begins to be the finest superhero movie ever made for the simple reason that it best captured the essence of the character. There have been some other fine ones, like the first two Spidey flicks did a great job, and I really liked the X-Men, but Batman Begins meticulously developed the character.
It does a good job handling the death of his parents and the effect it had on him. It shows how that started him on a journey of self discovery which ultimately leads him to adopting an identity as an uncoventional crimefighter. It lays out the logic behind why he uses the imagery of the Batman, why he has the equipment he has, and his relationships with the people who help him realize his destiny and carry out his mission (Alfred, Gordon, etc.). The acquisition of his marvelous toys is detailed as well, and made plausible. His psycholological makeup is accurately detailed, and his moral/ethical code rationalized. The actors they brought in, with the exception of Katie Holmes, were excellent choices. Luckily, Katie is out and Maggie is in, which should make this next one that much better.
It's handling of how Bruce Wayne becomes the real mask, as the real Bruce is Batman to the core, is the real genius of the movie. Spiderman is really Peter Parker, the mask and abilities allow him to act in a way he normally wouldn't. Unlike just about every other comic book character, Batman CHOOSES to be what he is, something which draws us in. It isn't thrust upon him in the form of being a Son of Krypton, bitten by a radioactive spider, the product of some experiment, etc. He is an entire human chracter with an inhuman drive. He is the product of his choices. All of this is dealt with by the film makers, and the end result is that someone who never read the comic book would have a very good idea exactly who Batman is, and why he's such a magnetic character to us comic book readers.
This leads me to The Dark Knight. By doing all of these things in the first movie, we have a fully developed Batman character for the second film. This means that the Joker, probably the best nemesis of any comic book hero, can be developed as well. Instead of being a cartoonish, over simplified characature, he can be built up as well. We can understand what kind of person he is, because Batman is a finished product. That doesn't mean that his development won't continue, but you know the character well enough already to focus more on the Joker, instead of splitting time as in Burton's film.
The Joker is the perfect arch nemesis because he is like a photographic negative of Batman, not only in their imagery (he Joker uses his imagery to disarm the innocent, while Batman uses his to scare the guilty), but also in their moral, ethical and psychological makeup. While Batamn restrains and controls his demons, the Joker lets them run free. At the same time, they are very similar in many ways. It is a kind of yin/yang relationship. The Joker is almost a physical embodiment of Batman's own demons.
Prsonally I loved the pick of Heath Ledger. Just as Bale was a spot on match for Batman, Ledger is a physical match for what the Joker should be: angular, tall, with the right kind of face and voice. In addition, his is a very talented actor, as is the case with Bale. Once he was picked, I knew that the film makers were going for the real Joker, just as they went for the real Batman. A stone cold psycho who creeps you out and scares you, as opposed to making you laugh, yet who utterly fascinates you. Nicholson was great, but that wasn't the Joker. That was Nicholson making his own kind of magic. The real thing is going to be in this movie, and I'm revved up. Michael Caine's dialogue says it all - he just wants to see the world burn.
I trust that this movie is going to be a great one.