I'd say the easiest group for PvE is probably...

Paladin Tank.
Warlock, any spec.
3x Elemental Shaman.

You can easily break all of them down to one button dps/tank spam.
And 3x Elemental can heal any 5-man content.
Everyone is a mana user, so you want similar buff types.

If PvP is a motivational factor, probably go with a Feral Druid as tank.
Since you can dual spec to Boomkin, for battlegrounds.. and run 5x ranged DPS.

If you want to run 5x Warlock or 4x Warlock + 1x Priest, that will be PvP viable.
And you'll probably clear most/all normals, at level or close to it.
But heroics, and the badge gear will probably be out of reach.

So it depends on what you want to do with the team.
I'm running a PvE team.
Two PvP teams (one per faction).
And several mess around with teams, mainly for tradeskills, but also for casual fun not caring about progression.