People are going to be running one of these three as power STAB.
Return - 102 BP, no effects
Double Edge - 120 BP, 1/4 recoil
Thrash - 120 BP, locks in for 2-3 turns, confuses at the end of the lock-in
Off the top of my head, Thrash looks like the best option for teams that would use Mega-Pinsir as a cleaner, when resists are out of the way or weakened enough to punch through with Thrash. It does seem like an inferior option to Return though, especially when you consider that Mega-Pinsir only has decent speed and would like to be able to use Quick Attack when it needs it.