Depends what kind of PvP
Some classes do better in different situations and mostly every class in the game will perform just as good in a certain situation
You have to be more specific
Random bgs?
Random 2s?
Random 3s?
Skype bgs?
Skype 2s?
Skype 3s?
World PvP?
See, there is master God mode class holy grail you are looking for.
Example,
Shadow priests can do lots of damage in bgs, but if you don't have some good tanks/melee dps to hold them from you and healers to heal the tanks/melee DPS, then you will be near useless and you would do better as a melee DPS.
So what is better shadow priests or ret pally?
Right, none of them, they are equally good depending on the situation.
I really hope you and everyone else fucking understand this.