warlocks = dots, so you can run around while dotting everyone up. Paladin has strong casts, but has to stand still for that. So that makes him not a good option. same goes for a resto shaman.

Priests and Druids both got hots to keep the warlocks up. The strenght of a resto druid is that they're very moble and can run away if needed. While multiboxing with the locks that isn't an option which makes your druid a relative easy kill. Priests can have more hits compared to a boxed resto druid and got Psychic scream. That's why priests are best imo. PoM can heal effective in a small group,renew for single target. Prayer of Healing can heal everyone up at once and is relatively cheap if you effectively heal everyone in the party(AoE for example).