MW and priests are both amazing.
Pally is also amazing due to his heals racking up an INSANE of dispels due to an ability that when a pally heals = dispels poisons (and more, I think)
Resto druids are also sick when they use the Nourish pvp talent (regrowthing a target also places a HoT on the target, and this stacks)
Resto Shamans are better in RBG this xpac then any other xpac before, but it still does not put them infront of other healers.
tldr: Any healer can work in RBGs