I would vote Druids out of those.
Resto is good for PvP or PvE, if you stack them.
Feral is quite fun.
Boomkin doesn't suck this expansion, so that's an option.

They're the only class that can be Tank/Heals/Ranged-DPS/Melee-DPS.

Paladins and Monks can be Tank/Heal/DPS.
Since you don't have Monks, that would be my second choice.
Stacking healer monks has been successful in PvP for some people.
Not sure how they do for the other specs.