For a 4x Shaman group, the best tank is probably a Death Knight.
Because of the 13% spell damage increase.

But I would go with a Prot Paladin anyway, since they're so much easier to macro and play.
At lower levels of gear, the pally tanks much better.

For PvP, you can either stay as Prot spec, and be the "leader" of the team.
As most of your opposition will target the front toon.
And Prot will be the hardest to kill, by a long shot.
Or you could dual spec, to Holy or a hybrid, and heal the team.
You'd want to be ranged as much as possible, so Ret isn't the best idea for pvp.

If you're willing to go with click, to cover your dps..
I'd go Paladin tank, 3x Elemental Shaman (DPS/Heals) and 1x Destruction Warlock.
But that's 3 classes, as opposed to the single class you asked for initially.