Druid or Priest. The other two have come a long way, but I am still not quite comfortable with them.
Druid: Takes a couple GCDs, but once all HoTs are stacked, my health bar isn't moving much anymore. Each time I check the tree's mana bar, I can't fail to notice that it isn't moving much more than my health bar.
Priest: Bouncier than the druid, but reliable burst and plenty of panic buttons. The least burdoned by group maintenance, leaving plenty of healing time to me. Guardian Spirit is the greatest healing spell in the game.
Paladin: Don't like my health going down significantly each time s/o in the group is taking big damage too - for whatever reason. Always feels like the pally is desperately trying to catch up with the amount of incoming damage. Having to kite makes matters worse. Like the burst though when they aren't bothered by anything. Plate and bubble means I don't have to worry about them as much.
Shaman: Their healing still seems to be all over the place. Least favorite.
Best duo for 10-mans is Paladin + Druid. There is perfect synergy between their healing styles. Having the pally cover the gaps until HoTs are stacked and taking care of spikes w/o the druid having to blow through all cooldowns is just plain awesomeness. Druid brings stability, even during movement, Pally delivers the burst. It can't get any better than that