OK, so we have Pinsir, Scyther, and Heracross, lets compare them.
- Scyther is the fastest of the three with decent attack but low defenses and 4x weakness to rock and Technician ability which boosts a Pokemons weaker moves specifically with a power of 60 or lower by 50%.
- Heracross has great attack, decent special defense but low defense and speed and 4x weakness to flying with the ability Moxie boosting its attack by one stage upon fainting an opponent.
- Pinsir has great attack and good defense but low special defense and speed however is not 4x weak to anything but is weak fire, flying, and rock and also has the ability Moxie.
I think that Pinsir or Scyther are the best options but Pinsir would be my choice because it only has three weaknesses and good attack and defense and with Moxie boosting its attack whenever it takes out an opponent while Scyther is fast but frail and will take a lot from stealth rock and not having attack as good as Pinsir's. However if you plan on evolving Scyther into Scizor then that's a good choice as well because Scizor has great attack and defense just like Pinsir but takes some speed for attack but If you don't like the speed drop then I suggest Pinsir as your choice.
I hope this helped.
