The one I had the most fun with was Specter. I played it more as condi DPS rather than a healbot, and I liked it. It felt good (which is the most important aspect of any class, if you ask me), and despite people saying it's weak, with the spec and gear I used it did a lot of damage, and was nearly unkillable to boot.
That being said, I don't really know if it's the one I'm most looking forward to. I play Mesmer a lot; I didn't like Virtuoso that much, but that may just be because I really didn't understand it, and I'm really interested in seeing how it's been updated. I also play Necromancer a lot, and Harbinger showed a lot of promise; it was just kind of...bland. But I'm also really interested in seeing how that one turns out next update.