On paper, he's the worst kind of villain: no solid backstory, no real motivation, no sympathetic qualities, no real grand-plan...
Batman's rogues gallery has some pretty damn sympathetic villains, yet the Joker is the one people seem to like the most.
WHY DO YOU THINK THIS IS, #OFFTOPIC?
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Because he's Batsy's foil He's the chaos to the order the crazy to the sane the smile to the frown