Rogues are wtfstereotypical and easymode, but a good way to learn the game a bit since they aren't that complicated? I'd say go with mage, though. I really should play mine more, still only 64/65 (I don't even remember).
Mages are one of the most loved DPS classes for all instances, though, because of water and sheepage. Crowd-control is always handy. And, AoE. I've never seen a group say no to a mage.
Warlock can be pretty darn nifty too, but I have yet to see a warlock kick as much ass as a mage.
Hunters are fun, too, but from what I've seen, they've lost their utility in end-game. If you're just going for kicks and giggles, though, they can be a lot of fun, too.