Priest is the easy mode 5 box healer. Shaman would be a great choice to add to your group of pally, mage, lock, priest just for the fact you would then have 2 viable healer/dps that you could choose/switch between. + the back up heals if needed.

I vote them in this order

Priest - Easiest
Shaman - almost as easy with more group synergy since you already plan on having a shadow priest
Druid - Druid
Pally - Possibly the least reliable healer for boxing IMO