The Joker.

"This capricious nature, coupled with his maniacal blood-lust, makes the Joker the one villain that even the DC Universe's other super-villains fear; in the Villains United and Infinite Crisis mini-series, the members of the villains' Secret Society refuse to induct the Joker for this reason. In the one-shot Underworld Unleashed, the Trickster remarks, "When super-villains want to scare each other, they tell Joker stories."