Voted Druid just to skew the poll
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The real answer depends on the group and the content. I'll take a dedicated shaman healer for groups where the tank doesn't have the hp to make cheal safe or efficient.
In hard content at higher levels especially with a beefy tank- cleric always wins.
Shamans are still damn good and they will always have more available mana. Toss slows into the mix and they are by far the most capable and efficient assuming you don't otherwise have a slower.
Druids are fine and I've successfully done chardok and done seb groups with them main healing, but they have obvious limitations. Bear in mind I was also barding these groups for mana ticks and up to 35-45 hp/tick hp regen. Without that hp and mana regen Druid might have struggled to keep up.
So yeah depending on content if put it:
Cleric > shaman > Druid
Cleric = shaman > Druid
Shaman > > cleric > druid