I think Warlock Hellfire is *the* highest AoE ability.
So...Warlock hellfire + Priest Holy Nova.

But...I would go with Mage + Priest just due to mage CC. Polymorph beats out the utility of a warlock in a group slightly. A warlock brings a lot to the table though, soulstone, healthstone, summoning, pets, etc.

On my 5-man team I have my AoE setup to Arcane Explosion on the mage, Hellfire on the warlock, Consecrate on the Pally and Holy Nova on the priest. The healing from the Holy Nova really helps when your clothies draw aggro during an AoE and in PvP it helps get rid of those pesky melee that keep getting into my group. Nothing funnier than a melee beating on your clothies and you start AoEing and they see their health drop and try to run away and get owned by the range on Arcane Explosion.