Joker: Vincent Gallo (Buffalo '66) would rock, but that's unlikely because he's notoriously difficult to work with. Lachy Hulme looks it but I don't know about acting. I know Sam Rockwell could do anything, so any of these guys would be good.
Harley Quinn: Brittany Murphy (Sin City). Gain a little weight and for some curves and she's a dead ringer.
Mr. Freeze: Ben Kingsley (House of Sand and Fog, Ghandi) is perfect. Although John Malcovich (Con Air) would be incredible as well.
The Riddler: Edward Norton (Fight Club) hands down. He's perfect for the cerebral Riddler from the animated series or from Jim Lee's comics. Jude Law is a close second, though.
Catwoman: Angelina Jolie (Mr. and Mrs. Smith). Come on, you all know it's perfect. Stop hating.
Mad Hatter: Martin Short (Prime Time with Glick). You know it!
Poison Ivy: Charlize Theron (Aeon Flux) could do it, though I don't know how good she'd look as a redhead. I'd prefer to find a young, sexy, REAL redhead, but for now she's definitely good enough to eat.
Two-Face: Guy Pierce (Memento) could definitely nail it. I don't like Liev Shrieber for the part at all. I just saw Manchurian Candidate and he's not a bad actor, I just don't think he'd fit really.
That's all I KNOW for now...all these guys would be kick ass.