Jervis Tetch or Mary Dahl. I really sympathized with Jervis because his origin for villainy is something not totally unbelievable. The fact that it was so plausible made it all the more terrifying. Same thing with Mary. How many times has a person looked in a mirror and just felt the inkling they didn't look like they felt they ought? In my opinion the best villains are the ones with believable beginnings.
"Look! That's me in there...the real me! There I am...but it's not really real, is it? Just made-up and pretend like my life, my family, and everything else. Why couldn't you just let me make-believe?!"
The Mad Hatter's pretty great, especially in TAS. I wish they would do more with him instead of the typical backstories he's always slapped with in the comics. It was nice to see him in Arkham City though.
He was originally in Green Lantern but then made appearnces in alot of other dc comics, but i have always liked Solomon Grundy. And then there is Hush, dont really like his character but the batman hush comics are just great.