I've found stacking Resto Druids to be very strong.
But running a solo felt weak, because of the ramp up time on the HOTs.

I've never had an issue with a Resto Shaman as my heals.
Stacking Resto Shaman will likely be strong too.

You're asking for a solo healer, not stacking several of either... so between the two, I'd go with the Shaman.
Personally, with the DKs I'd take a Paladin over either... more for their PvP potential, since they have enough heals for the PvE end of it.