I don't really like the Demon Hunter - I don't like the theme, the seeming uselessness of the discipline resource nor the evading playstyle. Lacking interest towards the Monk and the Wizard as well. Witch Doctor is by far my first choice, although I can't resist the thought of a nostalgic, whirlwinding Barbarian alt soon afterwards.
Monk, and I'm not sure why! I used to want to play it the most!
So far I enjoy playing the Demon Hunter and Barb the most, then Witch Doctor, then Wizard, then Monk. I'm not going to restrict myself to certain classes though and will definitely play them all. My opinion will probably change on each class later on when I can see what they can really do.
Barbarian first and monk last. Monk feels like a lesser melee class compared to the barbarian and I rather try the other range classes available. Sort of strange since I was so hyped about the monk when it was announced.