(one being the top)
5) Riddler-- He's very entertaining, but not very threatening. I loved his riddles and (especially) his interview tapes in the game Batman: Arkham Asylum. But... least of my favorites. Though he is pretty bad-a**. Plus, he has a great sense of style!
4) Harley Quinn-- Though many don't actually consider her to be an actual villain and more of just an extension to whoever she's working for, I find her funny and adorable. I prefer her original version, the 1990 animated series version. And if you remember, in the final season during the episode Mad Love, she comes unbelievably close to killing B-Man. Closer than many villains have ever come. I don't like how she and the Joker are "done" in the comics now. Still, some signs point towards them getting back together. Need I say yay?
3) Poison Ivy-- The feminist in me loves her so much. Despite the fact that she uses her sexual appeal to fool and take control of men, she's extremely pro-woman. The fact that she's always encouraging Harley to let go of the Joker and make her own route makes her cool. Not to mention her eco-friendly crimes. How exactly can Batman be upset that she's trying to save the rainforest? So she kills people! They kill her plants.
2) Scarecrow-- I've recently learned to love Dr. Crane. He is scary and dangerous. His fear gas could drive anyone insane. Not to mention, instead of just using the gas to get people out of his way, he uses it and studies his victims. How cool is that?
1) Joker-- How could the Joker, Batman's true foil, not be at the top of anyone's list? He is insane, genius, creative, diabolical, and funny. He can watch someone and discover how they tick. Just look at what he's capable of! He has taken a strong-willed, career bound women and turned her into a crazy henchwench! He has made Batman consider killing (and depending on the comic, actually murder). He has unhinged Gotham multiple times and has taken control of Arkham multiple times! He is a mad scientist, his joker gas killing and stretching people's faces into horrible grins and his titan chemical changing people and turning them into beasts! He is gay for Batman, yet straight for Harley (and plenty of other women whom he molests). His humor is scary yet actually funny! The Joker is wild and crazy, and that is why he works so well against Batman.