There are 7 fully evolved Fighting types available before Kanto in HGSS, as per the HearthGold and SoulSilver Pokedex - Heracross, Primeape, Poliwrath, Machamp, Hitmonlee, Hitmonchan and Hitmontop
Acquiring the Hitmon triplets requires all 8 badges and a tedious sidequest in Mount Mortar, so we'll leave those three. Poliwrath requires the Water Stone to evolve, which means grinding at the Pokeathlon Dome, which all may not want to do. So, we'll leave it to. Primeape comes later than the other two options and has nothing extraordinary, so we'll leave it too. We now have two options left -Machamp and Heracross. Remember that all further conversations about the Machop line will in regards to the traded Machop acquired in Goldenrod
Now, if you can't trade, it leaves us with Machoke and Heracross, where there's a clear winner - Heracross. It has a better BST (500 v 405), better Attack, Speed and Bulk. Use Heracross if you can't trade
Now, the matchup is much closer if you can trade. Let's compare them, shall we:
Heracross (Advantages)
1. Faster (85 v 55 base speed)
2. Additional typing (Bug) lets it take care of Psychic and Grass types
3. Better moves - Heracross has all-round better moves, with Brick Break and Close Combat being better STABS than Cross Chop (the traded Machamp doesn't get No Guard), STAB Megahorn and coverage like Shadow Claw and Aerial Ace
Heracross is harder to obtain and level up though. Heracross comes at a very low level, Headbutting trees is a game of luck and it has a slow Growth Rate without boosted experience.
Machamp (Advantages)
1. Traded, so gets boosted experience
2. Marginally Higher Attack (130 v 125) and better bulk (90/80/85 v 80/75/95)
3. Easier to obtain and level up
4. Is female, which helps v Whitney, comes with a attack boosting nature (Lonely) and has assured good IVs
(15/25/20/15/15/20) and a Macho Brace, helping with EV training
The only disadvantages it has is its lower speed and its best STAB being only 80% accurate. But Machamp's advantages far outweigh its disadvantages - It also gets Wake-Up Slap and Vital Throw for more reliable STABs, and Payback for covering Psychic types. It also has enough bulk to overcome its low speed, and Payback also perfectly pairs with it. It also is very good against Whitney, which is a big plus
TL;DR: If you can trade, use Machamp. If you can't, use Heracross
Machamp, Heracross and Machoke's DB pages
Smogon's HGSS ingame viability rankings - Both rank the same - in A tier