Druids and Paladins are great.
They can tank and heal, and DPS (both melee and ranged, in the case of Druids).

A lot of people love Shamans.
I see a fair number of Hunters and Death Knights around.
Not a lot of people go with Rogues or Warriors, although 4x Warriors with a Healer has been done for PvE stuff.

In general, tanking classes and healing classes are good to have.



Will your end focus be PvE or PvP related?
That's a big one on team composition.

For PvE, I'd likely go: 1x Paladin with 4x Shaman.
For PvP, something like: 5x Paladin or 5x Druid.