Shaman is strong for his mastery (healing low health targets), spirit link, fear protection big heals vs earth shield (which sucks if shaman gets trained cause WS is better for mana) and OP with 6s interrupt. Shaman does not have an oh shit button like bubble. With 4x DG you have peels so you might get away with it. But I doubt you can take advantage while boxing of the 6s interrupt that makes shamans so good.

Don't forget that the perception of shamans being OP comes for a good amount from their synergy with the lock.

Pallies output is nuts, bubble can give you time to land a kill and takes less micro management that shaman (provided you don't make full use of wind shear).

I'd personally go with disc, but thats more cause I dislike pally/shaman healers than for an objective reason.

Druid, well you might as well use 5 dks then.