Machamp's moveset in Pokemon GO and in the main series games has always been about one thing and one thing only: punching really hard.
Although Machamp might seem kind of simple in the face of some of the crazier new fighting types, it still holds the belt for best at just hauling off and punching things. Machamp is a Fighting-type, but the second you put an Ice, Dark, or Rock-type in front of it that Pokemon will be dust. If you want a no nonsense fighting type damage dealer, Machamp is your Pokemon.
Machamp Moveset Pokemon GO: Best Moves to Use
For Machamp, no matter if you're going for a defensive or offensive build, your best movest is going to be the same: Counter for the Quick Move and Dynamic Punch for the Charge Move.
Luckily neither move has the restrictions they had in the core games placed upon them in GO, so Counter can be used whenever and it's way easier to hit with Dynamic Punch. A Pokemon's moves can be changed by using a TM or by spending a set amount of Stardust and Candy.