Right now I'd say mages, but post wotlk I'd say Shadow priests. Spriests are being converted(pun intended ) into pure dps and lots of dps buffs coming for them. They also have dispersion, which is amazing for survivability(think banished yet regening 33% of your health), and psychic horror which is a 4 second aoe deathcoil, and also fade will break snares. It's looking good for shadow priests if they don't run out of mana too fast like they do now.