I`d say its Hunter, of course, mage are also very powerful, against melees, not so much against ranged, and they`ll get buffed in patch 4.3(Hunters Will get a damage increase), have you ever met one in arena ? It`s mad as hell, especially 3v3, and their survivability is very imbasauce aswell.
You might argue with me, but wait till you meet a great hunter in 3s, pain truck inc.
Take this from a 2.2k Achi(Ed) In 2v2 / 3v3 / 1.7k 5v5 Priest, theyre mad, mad as hell.
This is all from a perspective of a healer.