I'll echo the prevailing sentiment that my favorite healer is the healer that knows what they're doing. It's hard to say whether or not I have a favorite class, because I've been with good ones and bad ones of all healing classes. My personal favorite healer now is a 70 priest in our guild, who is phenomenal at what he does. I don't even realize I'm taking heavy damage, most of the time, he's that good.
Case in point, ran Durnholde 3 times with that priest one day. Then ran it once more with a different priest because he had to log. The first 3 times, we blew through it and took down the last boss with ease. The 4th time, we wiped on the 1st and second waves before the last boss, then we wiped on him, too, because the healer wouldn't mana up between fights. She also decided she'd rather heal her guildies/friends in the group before healing the tank for some reason. We managed to beat the thing once I got them to listen to my instructions and let me build aggro, but it was rough going.