Warrior by far. It's what defines Guild Wars. When you go to war, you don't want to bring anything else but a warrior. Some savage beast who will run into the heart of battle and tear everything up with any weapon he can get his hand on.
Monk would be next, because they keep savage warriors alive.
Hard one... I like to play Hammer Warrior for how I effective I can be in PvP, but I also like Elementalist to nuke everything and Mesmer to have fun...
I'll go with Elementalist, it's the one which I have the most fun with, even if I don't play it very often.
Ritualist. I don't know why, but she's always fun to play with. Though dervish is a very very close second. But really, I like all my casters and my preference shifts between them constantly.
If you like killing thing by hitting them, try an Illusionary Weapon Mesmer build... you'll wish your Warrior did 42 damage a hit no matter what.
I picked Mesmer, just because they are so flexible: IW, echo Nuker w/ Illusionary Signet, not to mention the standard counter-builds. I'm glad I dusted off my Mesmer and started playing Nightfall again, my Warrior was getting boring.
If you like hitting things, why not look totally cool while doing so? Plus, I'm against steroid use.
Second would be a Mesmer, since they can screw with anyone, anytime, and most people don't know how to play them. I guess I just like the underdogs lol.