There really is no correct answer for which class is the best FC because there's so many different factors that have to be taken into consideration.
What class setup does your team have? What class setup does the opposing team have? Who are the players on each class for your team and the opposing team? What class restrictions are present? What sort of strategies are both teams running? Are you queuing for a premade or just pugs? The list goes on and on.
Ideally, if all of the stars align I would go with a Warrior over a Druid or Paladin. Then again that's just my opinion and it's subject to change at any time.