On paper, he's the best villain. He has no real motivation other than to -blam!- with Batman, and he has no qualms about killing people. Pure evil. Random, chaotic violence.
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I'd say the best kind of villain would be one who has good motives that are arguably better than the protagonist's. How are the Joker's actions anything other than stupid evil-for-evil's sake?
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What can be more evil than random violence for the sake of random violence? There is nothing more evil than a man whose only motivation is to cause wanton random destruction and mass violence.