I had the same problem as you. With the intention of being a hunter but I realized they are both OP and overpopulated. What I was told is that priests and hpala's are what are the game changing factor atm. But from the choices you have provided, as Pizza said locks are viable when being healed, mages have burst damage and a great CC. And go resto if you want to heal and FC. It all depends on the position you want to fill while BG'ing. Whether it be DPS, Healz, or FC. I have taken all these into consideration and I chose to make a 19 pally. They can fill all roles, thus my reason for choosing that class. Whatever roll you wish to choose, I hope its right for you. GL with your New Tink!
Go Holy Priest for decent CC (chastise + psychic scream), good healing (renew, flash heal, PwS) and even some very decent dps (w 3/3 divine fury)
Among the listed I'd go for resto druid for the diversity and (at least comming) OPness, or lock to bring some diversity. Locks are ok now, and look like they will be better come MOP.