All time top has to be the Joker. He's brilliant, funny, scary and extremely deadly. He's the perfect villain for Batman. He’s insane… and honestly, Batman isn’t all there either. He has no superpowers and has to rely on his brain. So in those respects they are very much the same.
Then they are polar opposites. Batman does not kill, where the Joker kills whenever he feel like it. The Joker is garish where Batman is dark and brooding. It’s ironic that the hero here wears dark clothing, while the villain is dressed in a very colorful manner. Batman stands for law and order, while the Joker stands for chaos.
He’s just got an over the top personality. He doesn’t care about money or power… although he does indulge in revenge. He views life as a joke and death as the punch line.
A close second would be Magneto… because of everything he’s seen and everything he’s endured, he refuses to let it happen again. In his mind he’s a freedom fighter. He’s incredibly powerful and intelligent.
In a lot of ways I can identify with these characters’ motives… perhaps somewhere deep in my darker half… but I can understand them... although not condone their actions.