If you have ever read the Killing Joke [You should!] you'll know that the Joker [under the right writer] is a sympathetic villain. While he even states in his monolog that the tale behind Jack Napier may very well be a fib, the dramatic back-story given in the Killing Joke is not the reason why we should feel sympathetic for the clown prince of crime. The real reason is that the Joker is a lost cause. At the end of the graphic novel Batman contemplates what will happen if the two continue to fight, Batman tries one last time to see inside the head of his foe and begs him to call it quits. The Joker than begins to do what he does and tells a joke the story ends with the Joker bursting into laughter along with Bats. Batman knows at this point the Joker is a lost cause. The books fantastic, it's sick twisted and very well written. Plus it's stocked full of wonderful symbolism.

Because he's Batsy's foil He's the chaos to the order the crazy to the sane the smile to the frown

Part of it was the Dark Knight, which, to be honest is my favourite portrayal of him that I've seen so far. He's clever, resourceful, merciless, manipulative, cruel and enigmatic. But most importantly, he has a point to prove, rather than the standard villain who goes for money and/or power. He's also ironic in a way, that in trying to prove that the Batman is just as corruptible as the rest of us, he helps prove the opposite when Batman takes the fall for Harvey's crimes so that people won't lose hope. That's my opinion, anyways.

classic joker right there. no job is too big, and no job is too [i]small[/i] for the clown prince of crime.

He's unpredictable, insane and lives to torment batman for shits and giggles. I don't know hoe he escapes jail so easily though..Must have a inside man.