I would say Gnomes are pretty good for ally if you dont mind them being "verticly challenged", because their escape artist ability allows them to get out of snares and such in PVP but does not have much use in PvE. I am not against Humans though, as their Human Spirit ability is usefull as it makes their regeneration rate much faster, a usefull talent for any lock. I cant really speak on the Horde, as i dont have a lock on there yet, but i think the undead would be a good choice because their cannabalize ability is usefull for regaining health rididulously fast. But again, not having a Horde lock i really wouldnt know XD