I also prefer priests over all other healers. They're good for boxing because they have the best AOE healing. Especially holy priests. And smite spam's smart healing is good enough for questing if you dual spec disc.

Priest goodies useful for boxers:

Prayer of Mending
Circle of Healing
Prayer of Healing
Spirit of redemption (can save you from a wipe on bosses)
Lightwell (laugh if you want, its very useful in instances)