resto druids are the best healers in general, however priests are way more represent due all the hunter cleaves atm.

rdruid>disc/holy>sham>monk>pally (Sham monk might switch places, there arent enough high rated monks to get a clear view on this.)

As far as the disc/holy debate goes; disc is better for cleaves due spiritshell while holy is better for most other comps.
Holy is alot more instants than disc and alot more burst healing.

Holy is mostly played in 2s and above 2.2. Anything else is usually disc in a cleave.