I would go for priest definitely. Fear Ward, AOE healing and buffs the only downfall is survivavility.

I played Holy pally since day 1 and it's not that easy to heal a 5man team during AOE damage situation. You're only AOE is Beacon of Light and the Glyph of Holy Light(not that big actually). Flash of light has a HoT now when under Sacred Shield. Holy paladin is very effective in tank and off tank healing. Since you have a shaman/pally and lock, I think thats enough to substitute holy pally's improved blessing of wisdom

Druid has some good HoTs too and my guildmate always scored the highest Healing output in Recount. Plus Druid can be tank, long range and healer anytime. Revive is also a plus

Shaman is also decent healer plus additional stack totems and additional Self ressurect