1st place goes to paladin (just use aoe all the time, 1 button leveling) mobs fall like flies and you have 3skills that push the dmg of this skill without beeing locked to the globald cooldown
2nd place goes to warden in support spec (hit mob, pet finishes)
3rd place goes to mage&summoner in support spec, just run, hit everything and finish with aoe
and tak as many bloodlust as possible then you never have to stop
other classes do well too, for example engineer with device-falldown-spam, warrior with spin for win and so on but these listed are outstanding