I agree with Jeremiah's group comp. You could also switch out the ele shaman for a feral druid.

Paladin is the easiest tank to box.
I have been using a fire mage in my highest group but at 75 the mages dps is starting to suck.
I use a demonology lock.
Holy priests FTW! (disc until 60 [outlands] though)
I dont really like shaman. Their self resurrection after a wipe (long cooldown) is nice though when you're learning a dungeon). I would personally rather have a different class in this spot. My preference for the other DPS class would be cat druid, death knight, hunter, rogue in that order.

Many different mixes can work. If you pick a prot paladin tank and a priest healer you can choose any three dps classes and be fine. Especially if one of the dps can tank (dk, druid) or heal (druid, priest, shaman) in an emergency.