Michael Emerson won the Riddler poll, narrowly defeating Johnny Depp. Crispin Glover, Joseph Gordon-Levitt, Edward Norton, and Guy Pearce tied for a distant third. I think Emerson would be my preference, but I'm still surprised by the results.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.