The team is the classic trinity.

You have a tank, pally is the best imo for boxing.
At least the easiest to macro and play effectively while boxed.

You have a healer to keep the team up.
I really like a Disc priest in this role.

And that leaves DPS.
I'd personally recommend an Elemental Shammy as the DPS.
But you could go with Shammy healer and another caster as DPS.

I love having a shammy on my teams.
They bring a lot to any team, and synergize very well with casters.
The pally (tank) uses mana and threat is based off of spell power (even if most of theirs comes as a percentage of Strength).
Any healer is a caster, and benefits from an Elemental Shammy.

Play whatever is fun.
It's your team.