Druids can Wrath, mana free.
Shaman can Lightning Bolt, and regain a small amount of mana.
I suppose (Disc) Priests can Smite, which until MoP will auto heal whoever needs it the most.
The Paladin is (imo) the premier PvP healer... but is also my last choice for PvE healing.
No clue on what Monks will be like, but they're worth checking out.
Another consideration... what can they dual-spec into?
Druids get you Tank + Heals + Melee DPS + Caster DPS.
Priest gets you Heal Flavor A, Heal Flavor B + Caster DPS.
Paladin gets you Tank + Heals + Melee DPS.
Shaman gets you Heals + Melee DPS + Caster DPS.
Druids or Paladins, whatever their spec can count as a tank... which opens the dungeon finder.