There's plenty of room for argument on the topic of "best healer" in the game. And with some big changes coming to paladins in the upcoming 2.1 patch, the debate will only intensify. We've got priests with their versatile array of healing spells; paladins with their excellent mana efficiency and great buffs; druids with HoT domination and in-combat rez; and shamans with powerful (but difficult to use) totem buffs and self-rez as wipe protection. We could spend all day arguing the pros and cons of each class in a raid or small-group setting without coming to a conclusion!
But let's overlook the debate for now and just ask this basic question: who's actually out there healing you? A priest? A paladin? A druid? A shaman? A rogue with a good supply of bandages?