I'd say the FASTEST is definitely Frost. You quest each area fully, aiming to make each next area Green and Yellow at best quests. This way, when mobs are green, you can round up ~10 mobs with Ice Barrier up, Frost Nova, Blink, Blizzard 1-2 times. You will get crazy exp from kills and any quest requiring dead mobs is a joke. You can also solo every group quest up till Northrend.