Druid or Monk, Paladins are good healers but I think their DPS is a bit low compared to druid and monks, not 100% sure though. Really depends on the other healers in the comp to what healer would be better. If you raid with 2 other paladins, your gonna wanna go with a druid or monk.
Personally, I would say go with monk, but I could be a little biased, I found druid healing quite boring (tho haven't tried the new shrooms).
If you wanna be in control of who your healing, when and where. Go Paladin
If you wanna be 50% in control and 50% not in control, Go Druid
If you don't wanna be in control of who you heal for 80% of your healing, Go Monk (not as bad as it sounds :P)
Pally is the strongest healer. I'd take MW last, and thats coming from someone who mained it last tier. That being said absolutely none of this will matter until the endest of endgames.