What level do you intend to do battlegrounds at, and any particular battleground? Got a guild to help you gear up?

Mages - Nuff said but not much instacast action, excellent offense and defense and survivability
Warlocks - Instacast dots, rays, very useful pets that will lock people down, you could have one pet type per warlock for ultimate utility
Shadowpriests - Instacast dot (two if undead), manaregeneration, liferegeneration, healing and resurrecting, not much for survivability
Elemental shamans - Power, totems, very nice defense if meleed, recommend if you can stomach leveling and gearing them
Balance druids - Gigantic nukes with not much spellpower at all, bigger crits than Chyna's, stealthing healing resurrecting in combat etc