I vote Clayface, since he is awesome. Though I also like Killer Croc, Mr. Freeze, Zsasz and Hush. Bane is cool too, he did break Batman's back after all.
He's/She's from Batman: The Long Halloween; was a mob-related killer whose murders revolved around holidays (Christmas, Halloween, Thanksgiving, etc.) His/Her identity was never officially revealed.
Not a loose thread. They did it on purpose. Its pretty much obvious. Its actually more than one person, the biggest being, well........ read it, its great.