It's hard to nail down to one favorite. I can nail it down to two: warblades and wizards. I've always liked fighters and wizards, and I think the warblade just does it better. I like the fluff, I like the freedom of builds, and I like being able to simply smack someone in the face. My only complaint is that some maneuvers are so much better than others that I end up taking them for all builds. I have a similar feeling with wizards. I've always liked the tactical aspect of the game, and I love having a handful of choices in front of me trying to figure out which one is the best. Again though, some spells are so good that I always take them.
As for least favorite, that's too tough to nail down as well. If I had to pick one, it would be the CW Samurai. It was poorly executed in terms of game mechanics, and the developers narrowly focus your fluff to say "This is what a samaurai is. Don't think outside of this stupid box." Just play a warblade and be done with it.