For their skills and the possibilities they allow, I can't pick a favorite out of them all. So, for secondaries, all of them!
For primary, I've played a fair amount with Ranger, Mesmer, Necro, and Monk so far. I enjoy them all, and I'll likely enjoy the others when I get around to starting them. Ranger was first and the title holder, so she gets the most experience. Still, I wouldn't say I like playing the Ranger more or less than I like playing the others.
Mesmer is extra special for having the biggest gap between (generally) expected and actual potential ... thanks to Dumbam.

(Though I still wonder why ... if he's the reason so many think mesmers are useless, how come more people don't think monks are useless thanks to Alesia???)
The only class I've tried as primary and not liked is the Assassin. I know! I agree! Done well they can be just wonderfully evil in all sorts of ways ... but the way they run ... oy! It drives me nuts to see that in the center of the screen all the time - I just can't get into playing it myself.
Luny the Undecided