Holy paladins survive forever. With Protector of the Innocent, I can stand there and spam heal a DPS while being attacked myself, and neither of us will die. Sure, with enough focus I'll go down, but I can last a very long time.
While I haven't played a shaman myself, I have seen them in action and they are just as good, if not better. Earthshield is incredible, and earthbind totem is HUGE in BGs. Not to mention Healing Surge is very powerful, very fast, and (as far as i know) inexpesive.
As a priest, Bubbles are really strong in this bracket too. AoE Fear is great, especially on such a short CD. I have seen quite a few Healers go Shadow Spec for Mind Flay, as the speed reduction is awesome for BGs. Plus fort is always a big help.
For druids, I can't say much here. I haven't seen many resto druids, and the only things I can think of as beneficial are travel form for running flags, roots to stop EFC, and HoTs for on the go healing. But if you are going to be running flags you should go feral for survivability, and I'm not sure how effective HoTs are in BGs because of all the burst.
In the end, go with what you think will be fun. Each class brings plenty to the table, so it's all what you will like to play the most.