Definitely Mage for DPS.
If you wish to heal, then go Priest...but I personally think Shamans are the best heals (just from personal experience).
I like Warriors, but tend to use them for tanking...
Mages murder DPS in everything, and in BGs they're very OP.
Priest heals are good too (I can't say much for them as I don't have one.. but my brother has one and he enjoys it). I do know, though, that priests KILL mana (holy/disc AND shadow). It *still* takes a LOT of downtime to drink, which can slow leveling AND dungeons.
If you wanna DPS, definitely go Mage.
But the problem is, for dungeons and such it takes forever to get in one as DPS.
Unless you're just gonna quest that is.
I have a Mage and I quest/dungeon, and I leveled her so fast!
Basically it's up to you, but I think if you do dungeons and you rolled Priest for heals you'd definitely level faster.