That's depend on what type of pvp You have on mind. The best solo pvp vocation is Royal Paladin, because of his highest possible combo damage. If equipped with The Devileye he can easily take out 200+ mage on one hit (if a mage doesn't use magic shield).
Mass pvp I would say Elder Druid is most effective, mainly because of his paralyze paralyze, wild growth and high SD rune damage.