Well there is two major sides to this piece, the best type of healer for Elemental Shamans and skill plays a major role as well. I've chugged through probably 20 different healers for Arenas trying to find the best fit for me.

I would say the weakest healer is sadly the Priest. They have nice AoE heals, a good hot and nice flash heals but they are super squishy, and I would have to say the worst mana efficiency out of all the healers to choose from.

At one point I felt Druids were the weakest class for me, but my mind soon changed when my Druid friend Drwurm and me pulled a 41-2 stomping session going from 1782 rating to 2006 this week. We just foundour synergy and some of the best tools they have come in the form of him stealthing and pouncing a target, locking it down in place long enough for me to take out. Past that they can throw out an NS heal and then another quick burst heal.. but the name escapes me at the moment. Being able to kite and avoid damage all together it really depends on the skill level of the druid you have.

Shamans are awesome, the extra grounding totem, quick NS heals - nice chunky Chain heals combined with a Mana Tide when things get drawn out is a huge plus, their mana efficiency isn't that amazing but the Mana Tide helps. I went 15-0 this week with the one in my video Bubbajiggles, he is flat out an awesome player through his "Self Awareness" which means he knows everything that everyone is doing on the battlefield helps immensely.

Last but definitely not least is Paladins, the BoP on you is clutch if they have a ton of melee, pretty much guarentees a victory. Bubble is huge against 4 DPS teams so they can throw out huge heals without being bothered for the initial burst fest, and their mana efficiency is insane. If the opposing team has a ton of casters though you may be in trouble due to grounding totems getting eaten up, allowing easy access to lock down your healer.

The most important aspect though is that your healer is rarely ever targeted first, most of the Paladins I play with switch into almost full PvE gear so their heals land harder. The determining factor though inevitably will depend greatly on your playstyle and how skilled your healer is.

If I had to sum up it would be Priests are the weakest, and Druids/Shamans/Paladins are the best options, each one has it's pros and cons so not one will be the best, player skill will make the deciding factor for you.