Depends what you wanna do as healer:
Disc - Big Heals, Big Dmg nothing more
RDruid - Good heals, viable fc, Kittydamage /purr
RShaman - Biggest Heal, best support-class ingame, no dmg though, possible fc
Hpala - Mediocre Heals, Some dmg, most op- and underrated debuff ingame, 2 "oh shit"-buttons
MW-Monk - high and constant HoT, fun gameplay, no dmg though, great moveability, possible fc