Here are my rankings in order of best performers:
Priest - Top healing and damage as a healer. sw: death is essentially an execute, really good for finishing off low health enemies. I've seen versa stack and they are hard to kill. Lowest playstyle limitation compared to rest.
Resto druid - I never played it, but from what I see, they dish out good dps and heals, they carry games because they are amazing at objectives, i.e. capping bases + running flag
Resto shaman - x2 quick cast to help top off team mates thanks to incredible mastery, able to counter spell and provide slow. Can push damage from afar
I play both hpally/mistweaver and I feel, they are best played defensively
HPally - Plays like a resto shaman, no ghost wolf but great survivability from bubble/lay on hands, hammer of justice is the only cc. Lowest damage amongst healer class, should focus on healing. Cleanse is great, but under utilized in this bracket.
Mistweaver - Gets punished the most from bad positioning, as they are the squishest, but can definitely top healing charts. Has AOE melee stun. Damage can be catch people offguard, but it has to be melee range. Generally should stay away from fights as they lack defensives compared to other healers.