Because I can't pick just one, but I don't have time to do them all.
1. Mozenrath- With him it's a combination of factors. I love the way he is animated, pale dark hair, like a Gothic villain or a vampire. Johnathan Brandis brought a lot to his character giving him a hidden bit of insanity as well as a dry wit. He just carries himself as a really dark mysterious character.
2. Warp Darkmatter (Hey, this is BLoSC forum right?)- Deifnately one of the things I like the most about him is his voice. I love the way Diedrich Bader gives him that sarcasm to it and the comments he makes, like he's a bored jaded Gen-Xer and an insult comic. I just love his more sinister attributes and I also love how he is shown with more dimensions, saving Buzz's life twice in the series.
3. Foulfellow- I just love the way he carries himself as one smooth operator always manipulating the conversations for his own gain very controlling.
4. Ichabod Crane- There is just something about him that is almost charming how he goes from this gangly geeky youth, to a charmer of the ladies, to a totally panicked person.
5. Alice- Well, she was always one of my favorite characters, but I always liked her stubborness, and her temper. And also, that kind of dreamy quality she has where she's not scared of the strange world around her, but still offended by it.
6. Queen La- I find her an interesting character, partly because of her looks (she has this innocent blue eyed look that one would think would be more of a good character) and then she has this voice that invokes evil. definately a study in contradiction