Would have to be shadow priest for me - stam buff, mana back, hots, 5% bonus to casters, shadow weaving which adds 15% on to locks' damage (although i think that's about to be nerfed, and scorch will be improved).
As Gnarlet said though, a shaman is also fantastic to have around.
Guess the key to good raids is having the right combination of classes and good groupings within the raid on a fight by fight basis.