Ya healer is player choice but I am thinking restro shaman as chain heal always hits the right guys who need it the most, riptide and earth shield take no skill. Priest and Druid (with the offtank ability of Druid as stated) are good of course but chain heal is the winner to my mind. Why make complicated grids and require you to determine who has the lowest health when chain heal does it for you?

If you only run 2 then ya healing maximization would be easy with the priest, but if you plan and 5 or 10 boxing the ease of the shaman is best (although a circle of healing priest will work also).