I think most people would agree that a paladin is #1. Consecration + Heals + Bubble + Res + Ret Aura

I would have to say that #2, for low levels would be a holy/disc priest with a mage/warlock being #3. Priests have an AOE that heals and damages. They can heal, res and bubble. They can spec into a bubble that deals damage. Fears work great to pull more mobs. If you're running with a mage/lock, you can't res (ss being the only exception) the lowbies, but SoC and AOEs are pretty effective as long as you don't get overtaken by mobs. I used a 70 holy priest to run a twink rogue thru the deadmines. Took about 7-10 minutes per run.