nobody really knows and we're all theorycrafting at this point. So far the three favorites seem to be shamans, DKs, and pallys. I image we'll see some hunter teams too, and they may be dominate in large scale PvP BGs (e.g. AV), but they will need healing. I'm always a fan of priests, but I jumped on the melee boxing bandwagon - it's just easier to melee than to use casters.