It's gonna be easier to start with what I think wont be that powerful.
Druids - Bad synergy with talents/gear, requires forms for feral abilities.
Mages - Too many school/spell specific talents
Warlocks - Destruction and Affliction have bad synergy and buff two different types of damage.
Priest - 2 healing specs and no synergy for damage, best healer but wont be able to kill anything.
Warrior - Good gear synergy but stances prevent maximum potential, no long term survivability.
Shaman - Very good synergy between enhancement/elemental but both use different gear, far too squishy.
This leaves us with:
Hunter/Rogue - Amazing synergy between specs for damage, same gear, best classes for one shotting.
Death Knight - Massive amounts of defensive cds, self healing and damage.
Paladin - Triple spell power conversion, stupid healing/damage. Immortal with a shield.
Out of these I think paladins have it best. It's hard to argue with 3500+ SP in full ret gear, much stronger than just holy paladin healing and still being able to do good damage.