For me it's because he's so sympathetic compared to all the other villains. Almost to a point where it's hard to see him as truly evil.
It's true, aside from that movie in 1997 i won't name, he is only doing these crimes to save the one person he loves, he never deliberately kills anybody in the comics, and he is usually always helping another criminal that promised him money in return for his services. He is just a man who will do anything for his wife, even though we know he will never find a cure.
it's kind of sad to think about, but his characters theme is all about love in the end.
Batman totally has the best set of villains of any superhero.
Two Face, Scarecrow, Riddler, Penguin, Catwoman, Mr. Freeze, Clayface, Killer Croc, Bane, Ra's Al Ghul, Hush, Mad Hatter, The Ventriloquist (one of my personal favs, lol), Black Mask, Poison Ivy, Harley Quinn, The Royal Flush Gang, Deadshot, there are just too many to name.
Typically, I like the villains that follow the Dick Tracy pattern, sort of that deformed, gimmicky gangster style. Penguin, Riddler, Two Face, Scarecrow, Black Mask, The Joker, those types.
Besides The Joker, I'd say that my favorite it Two Face, for the tragedy of his origin and the whole gimmick lending itselt to that crazy gangster Dick Tracy motif I prefer.
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