Personally...

I'd go Arcane Mage, Destruction Warlock, Elemental Shaman and Holy Priest.
With whatever your tank is.

And probably dual-spec both the Shaman and Priest, so you can mix and match.
Some fights the Earth Shield and Mana Tide might be worth more.
But in general, a Priest is an excellent healer for a team.
And Elemental adds a lot to predominantly caster groups.