The Machamp that I'd use as a defender is the one with low IVs that I picked up from a raid and am willing to transfer once it faints.
Otherwise, BP or Co with HS would be my pick. Typical Machamp counters resist fighting-type damage, and, while difficult to dodge, Machamp won't last long enough in a gym battle to get a CC off. All in all, though, don't put Machamp in on defense. Even Slowking is a better option in and amongst the usual suspects.