I am Blue Harvest
When it comes down to it, no idea's too Farfetch'd when it comes to building a pokemon team.
Also, mega-evolving Pinsir first turn is a big no no most of the time.
Much of what I wanted to say has already been said, so I'm throwing this out there:
Feint is good on M-Pinsir
Fury Attack makes for good wallbreaking
Quick attack is pointless when you have Feint
If flail gets stab bonuses (I don't think it does), then your opponent isn't surviving that.
Meet Tropius, the once good Grass type nerfed in X & Y
To be honest, I'd still rather just spam Earthquake against Aegislash rather than try to hit it with Feint as it uses King's Shield. Worst case scenario, you just don't do any damage for a turn, and at least you don't run the risk of mispredicting the King's Shield and giving your opponent a free turn to slam you with Shadow Ball or something as you hit them for pitiful damage.
Pinsir is just about the purest form of a Generation I pokemon you could ever come across. In it's normal form it's a very strong pure bug type with an awesome albeit sort of predictable move-pool (Earthquake, Stone Edge, Sword Dance and X-Scissor) with nasty attack power and decent speed to back it. Unfortunately for me looking at it's Mega Form actually for me at least makes it harder to use because there are so many better options out there with much much stronger and wider movepools but it has an amazing attack stat which no-one's walking away from without a scratch but it's speed is sort of on the borderline of viable and not viable.
It's 4 abilities though are all pretty awesome starting with Hyper Cutter which prevents intimidate users which wouldn't be a problem except most of them like Salamance, Gyarados and Arcanine all have a type advantage over it.
Mold Breaker is probably going to be the preferred choice as it makes most of the Pokemon resistant to Pinsir's attacks and effects (especially levitate users) vulnerable to what is a very powerful movepool.
Moxie for me seems to be a choice of either that or sword dance if your going for a sweeper strategy though personally I'd pick this because I would want to hit hard without leaving Pinsir around for an attack it likely won't survive.
Aerialate for me is a weird one as I've not actually seen it in action but it just seems like a wasted ability on Pinsir who's typing seems to match it but movepool doesn't.
I'd pick the normal over the mega 9/10 times when using Pinsir.
Mega Pinsir is one of the better Mega Evoluions out there, and the only reason Pinsir is even remotely viable in OU right now is because of its Mega Evolution. If you're using it in OU and you're not using its Mega Evolution, you're wasting a team slot. It's as simple as that. Aerilate is a huge reason for Mega Pinsir's success, since it turns normal type moves, which it has a lot of (Return and Quick Attack being the most noteworthy), into high-powered STAB moves. Return becomes a base 132 power move with perfect accuracy and no downsides. Quick Attack turns into a base 52 power priority move, just a little shy of Scizor's Bullet Punch. In addition to higher attack, Mega Pinsir gets added bulk, and a much better speed tier which lets it outspeed the myriad of base 100's in the tier.
If you're picking the normal form over the Mega Evolution, you're probably better off using another Pokemon altogether.
Yeah, regular Pinsir is horrible, lol. Even if you did wanted to use it for whatever reason, literally the only thing it can do that Heracross can't is use Stealth Rock. Any offensive sets you might want to try are far outclassed by Heracross with its stronger Bug STAB, secondary Fighting STAB, better typing both offensively and defensively, lack of a Stealth Rock weakness, and more rounded bulk (and regular Heracross isn't even that good outside of UU). Like KillerDraco said, Mega Pinsir is literally the only reason anyone gives Pinsir a serious thought these days. I don't understand how you can call regular Pinsir's base 85 Spe "decent" and then say that Mega Pinsir's base 105 Spe is borderline unviable when it's actually quite good. You outrun the vast majority of the unboosted metagame, including top threats like Garchomp and the Mega Charizards. Even if the opponent is faster, +2 Quick Attack is enough to OHKO many faster Pokemon such as Deoxys-S, Keldeo, Greninja, and Gengar, and even some such as Terrakion can be KOed after some prior damage. Mega Pinsir is one of the most dangerous sweepers outside of Ubers. Regular Pinsir is garbage. Sorry to be so blunt, but it's true.
I was talking about the +2 Atk boost after Swords Dance, not Quick Attack's priority, which is irrelevant to the point I was making.