Well, priests have very effective AoE heals, if specced holy.
A disc priest has a good amount of survivability with Imp. PW:S, Reflective Shield, focused will, and pain suppression. As well as having, some very good sustainability in the dispel-fest that is Arena. If specced correctly and with pain suppression up, you can have 85% resist to dispel for 8 seconds. You'll have two great buffs- Fort and Divine Spirit, as well as one of the more deadly PvP skills in mass-dispel, which can be reduced down to a 1.5 second cast if specced correctly. While you won't be as sustainable as a paladin, you'll be close, and can be a far superior healer.
As for a Druid, in my extensive experience, I've found them to be inadequate PvP healers just because of the nature of the beast. Most of their heals are heal over time spells, and they have little AoE healing power. However, they do have some very effective PvP skills in Faerie Fire, Hibernate, Innervate, Cyclone and Entangling Roots. A feel that a feral druid is more useful in PvP than a restoration. I am sure, however, that people have had good experiences with Druid healers.
IMO, Paladins and Shamans make the best PvP healers. Paladins have amazing staying power with bubbles and plate armor as well as some "oh shi't" buttons. Shaman have an unbeatable buff - Earth Shield - as well as some amazing AoE heals, but have less effective quick heals than a Paladin or Priest.
Ultimately, any class capable of healing can be good in PvP or PvE. It is all up to the skills of the player controlling the character. Just pick the class you think you would like the most. Maybe play each of them to level 20 or so to get a feel. I'll outline a basic style for the classes you're interested in.
Druid - Good utility... they're a hybrid class. and mark of the wild! Uses lots of HoT's, which if used effectively could be amazing group healing. Has some decent CC but mediocre survivability (no armor bonus in tree form and slower movement). Druid healing however, is all about utilizing your hybrid abilities. I would suggest if you roll a Druid, try a few hybrid specs. Test what works for you.
Paladin - Amazing survivability as well as great single target healing. Bubbles are nasty and their blessings rule.
Priest - Possibly the most complicated healer. You can't just stand their getting smacked on by two arms warrior, casting the same spell over and over. You have to adapt even more than the other classes, and it is vital for you to choose the correct gear as well as the correct spec. Can have very good survivability if played correctly.
Shaman - My personal choice of healer for an arena team. Has an amazing buff - Earth Shield - which may get buffed soon. Has great AoE heals, and good survivability. Weaknesses are lack of CC and inefficiency of quick heals.
But honestly. It's up to you. Everyone comes to these forums asking what class they should play, what race, what spec etc. It's a game, and games are meant to be fun. Play whatever spec is fun for you, even if others say it sucks. All we can do is give you information.
Also, try looking at the FAQ's for all of the classes you listed, as well as doing a google search or even looking on the O-Boards. There is a multitude of information just waiting to be found. How do you think the people that answer questions like these got to the point that they can answer them? We tried things, we read things, and we had fun. Do the same for yourself instead of immediately looking to other people to give you answers.