Monks are definitely the worst in 2s. Warriors and Mages are counterable, but their upfront burst is so painful with their level of control that a good one can really ruin your day. I've yet to come up against an Affliction Warlock/Shadow Priest at my MMR that has done anything particularly painful.
It crazy how many people are lumping destro/aff lock as if they are the same spec. As others have mentioned its more about what type of pvp you are doing. I think power is in a pretty good space actually but if I had to choose I would have to say monks. Locks are amazing because of spam able fear, Aff is amazing because they have the best spread pressure and destro is good because of the high burst/execute and self heals but I dont think its over powered. The only reason they may feel overpowered is because of the synergy they have with with their team members but this is an over powered class thread not an overpowered comp thread