My answer to the origin of Joker
It is obvious from my profile pic that I am an avid lover of the Nolan Batman Universe. But nothing more so than the portrayal of the chaotic Joker. He is, in my mind, the perfect villain. Never having any regrets or second thoughts about what he does. Always having a sound strategy, and is always someone to be feared of, even when he is in the clutches of the Dark Knight himself.
The fact that he is the way he is should be enough for anyone to fear him with every fiber of their being. But what is most important about him is the fact that no one has any idea of why he is the way he is. It is a complete mystery to Batman, Oracle, the writers; even the Joker himself has no idea why he is the way he is.
I feel Ledger captured this perfectly across the movie and actually allowed me to like him as an actor. Not because the role essentially killed him, but because he brought that chaotic character to life in a way unimaginable to most.
While it may speak odd things about my character, but Joker is one of my favorite characters to come out of any written media because of these reasons and I feel proud knowing enough to share my love for this character in answer form, and I also have plenty to still learn about him and his various alias' (upon my 3rd viewing of Dark Knight, I realized that Melvin White is one of the alias' he used during the comics; for example)
Most of my love for Joker comes back to a childhood question I had while watching the typical cartoons wondering "why does the hero always win?" Being introduced to the Joker for the first time was a happy moment, discovering a villain that didn't just get beaten in the end, but no matter what, had the last laugh. This was furthered for me when I learned of the Jason Todd arc of the Batman comics when it was decided (by the readers, but still powerful regardless) that Joker had killed Robin (Jason Todd at the time; though he would come back as Red Hood, Joker's first known criminal alias, in a strike of irony).
While this answer is not wholly representative of my Joker knowledge, I feel proud of it being a highly voted-answer and hopefully spark some more readers interest into Joker mythos, and appreciation for one of the most perfect villains conceived by man.