They all have their ups and downs, historically there are some that have done better, mages have pretty much always had a solid place in pvp, priest have almost always been a pretty decent healer. They all change though. Shamans used to be incredibly OP in Vanilla, Spriest used to be incredibly OP in parts of TBC, Warriors and Fire Mages and Paladins of all specs utterly destroyed in different parts of WOTLK. they've all been bad, all been good.
warrior and mage... well and rogues