10. He's Kinda Similar To The Joker
Not that Batman's greatest villain is all that original himself, but you gotta admit he's memorable. So memorable, in fact, that it makes the similarities between him and The Riddler obvious to anybody with half a brain. All of Gotham's greatest threats have their own weird gimmicks, from The Penguin's...resemblance to a penguin to Clayface's...clay face. Man, they really do have some literal names, huh?
The conceit of both The Joker and The Riddler is pretty similar, however, and we're not just talking about the green-and-purple colour scheme. Well, we are sort of talking about that. They're pretty similar. Both characters have a weird history, as each was introduced as a one-off bad guy in the Golden Age (thirties/forties) incarnation of the Batman title, and rarely seen after until they were brought back properly in the silver age (fifties onwards). So at the time the fact that they're almost identical wasn't so egregious - they were a couple of wackos who happened to have a similar obsession with both the Caped Crusader and carrying out crimes with a particular linguistic bent.
The similarities haven't stopped there, though. There's the punning "real" names for both villains - Edward Nigma and Jack Napier - and both turned to a life of crime simply because they were super-intelligent men who were bored. Their intelligence and knack for manipulation has also alienated both from the rest of Gotham's bad guys. They're both bonkers - which isn't that unique amongst Batman's enemies - but each has their specific form of madness, a fixation on the Dark Knight and with constructing elaborate crimes.
Look they're basically the same except one has never had an Oscar-worthy portrayal on film, m'kay?