druid - king of aoe heals, if u want to fill your screen with green go one of these bad boys...
paladin - king of keeping tanks up, enjoy spaming 1 butten the entire time, taking forever to regain 40k mana but hardest hitting heals in the game and tanks depend on it.
sham - fastest heals in game once u get haste stacking... can be fun for some, most interesting healing style imo need to know when to use what heal not to faceroll heals...
dis-priests - shielding as often as u can, pennance on cd fill with renews and flashheals
holy priests - kinda feels like a mix of pally/sham
i have personaly played all healers, and they all have their strengths, least of all i enjoyed was pally healing, most complicated would have to say sham healing, most fun would def be resto druid thou :D
hope this helps a bit