MM, well, you see the number of threads. Honestly I feel it’s a bit exaggerated but MM is great for BGs. Arenas, they struggle a bit under the larger amount of pressure.
BM I think is doing well all-round, due to it’s simplicity, pet utility and still solid damage. Also swarming enemies with huge pets infuriates others. Including me. 
Surv is the most difficult spec to use well, and is melee so epic BGs are an exercise in masochism, but a good Surv player is downright terrifying.
Really think it boils down to what you know how to play, and what you enjoy playing! Think any of them can do work.