When he's written well (which is unfortunately exceedingly rare these days), the Joker. Harley too, though BOP and Harleen are the only good things for her in recent memory.

Beyond that, love Two-Face, Catwoman, Freeze (BTAS), Ivy, Riddler, Scarecrow, Penguin, the Hatter (BTAS) and Hugo Strange.

And although a controversial take, i like Talia better as a villain. And she's way more interesting than her father.