Shield Tank/Brute and a shield scrapper, since you can synergize grant cover to help each other and the team with extra defense.
Defender, Corruptor, Controller or Dominator with Powers that buff resists since those shields will have their defense above the soft cap.
4th could be An Aoe Centric Blaster, Sentinel or Mastermind.
Really, any Combination of AT's working together is going to be very formidable.