8. The Joker
This was hinted at in The Dark Knight, but a commonly accepted truth in the DCU: The Joker is the villain other villains are scared of. Be it his mania, his vicious approach to the world, or perhaps the sheer unpredictability of his actions. No one has topped the Joker in sheer quantity of atrocities committed. The Joker will inevitably appear a fourth time on film. The previous three appearances, however, never had the Joker fight against the Justice League. There are two particular stories of note where the Joker held off the entire DCU on his own: The Last Laugh and Emperor Joker. Both could potentially be great stories. Not only that, but the Joker is great when he's bouncing off of Lex Luthor. The two aren't pals by any stretch, but their onscreen chemisty is always brilliant. Two of the greatest villains ever working together. That is the making of a great story.