In a full healing roll it has to be a disc/holy priest. It's not so much that the priest is the most efficient, or even the best healer - it's the survivability of it. Chuck in improved inner fire, PW:S you are essentially trying to take down a mail class with PS as well.
My preferable option is actually a holy paladin. This is not a PUG however, and we have skype in both BG and arena. The uninterruptable healing (kick immune etc - can't remember the name), HOF, flash healing and absolutely INSANE lastability make this number 1 for me. The lack of healing on the move is not such a big problem if you have VOIP, as you can always get good positioning. In a PuG i'd take the priest however.
Druid is good, but survivability, particularly after 3.1 isn't hot. A rogue with sprint and another cd will burst you down fast.