I like em all actually.
In WoW, I played a feral druid, which is primarily stealth (thief), does awesome damage and can take big hits (warrior), and can still cast heals and damaging spells (wizard). I tried other classes as well, but they were never as fun as my feral.
If I HAD to pick one, I'd probably go with the Warrior, particularly the Paladin. Why? Because the Warrior is the one archetype that can hold it's own in just about any situation. The Wizard is too squishy at times and has to rely on their mana, and the Thief cannot always be completely stealthy or get the drop on their opponent.
Besides, I'll never have as much fun in a game as I had with my smiterdin :3