If you actively play the tank, and have the shammies as your group/slaves... any of them will work well, so it is a preference call. As far as instances go, they're all very viable choices.

If you plan on pvping with them, the Paladin could go Holy and be your healer and the Druid could go Boomkin as an extra DPS or Resto as a healer. The other two DK/Warrior would play melee while your team is probably ranged. So if you want to PvP, I'd suggest a Druid probably although the Pally works too.

My preference would be Paladin if you PvE mostly or Druid is you do both PvE and PvP.