As somebody who plays all the healer classes I have to add that monk is by far the easiest. It is practically immortal, I hit 2k+ with it while I was 500 ilvl (when 522 was max), it has excellent mana control and very strong heals/shields. The only weakness is cc but not many people know how to actually cc so if you're good at it, you can beat any combo.
And what the hell made anybody here think that disc is a bad healer? Sure the shields can be dispelled but they practically have no cd and purge costs a lot of mana so it's not exactly affordable. As Mayfire said, the shields are very strong, it has good cc and decent heals as well, I'd even dare put it as the second best. And holy priest sucks in arena, it simply cannot heal enough, can easily be nuked down by any above average dps.
Also WolfCD, retail means NOTHING here, seriously, who cares about retail, we're talking about warmane healers. I've only seen only holy priest above 2k, and I forgot his name...