These are good pairings for different classes and who they should shoot for when questing or grinding. Not that this is for PvE, not neccesarily suggestions for PvP arena teams or anything.
And this is also the most efficient pairing from my expierence.
Priest: look for rogue, druid, hunter (with dps pet)
druid: Since druid can do both DPS and healing almost seamlessly, a high dps class would be best.
warrior: Priest, possibly dps character if the enemies are a few levels weaker.
rogue: war,druid, or priest. paladin is best
shaman: virtually anything
mage: war, war, war. Possibly an experienced priest
hunter: DPS character, but have your pet taunt often.
paladin: since its basically a priest and a war combined, it can go with everything, especially druids, warriors, priests, mages, or best of all, rogues
warlock: Since it has such a wide variety of pets available, it can adapt with any partner. Although a warrior might be best.