Machamp can use both Abilities very effectively, and it's all a matter of what is needed most on your team.
You could always use Lead Machamp:
Machamp @Lum Berry
No Guard Ability
Adamant Nature; 240 HP/248 Atk/16 Sp.Def/4 Speed
~DynamicPunch
~Payback
~Bullet Punch
~Stone Edge/Ice Punch
However, on the Guts side, there's Tank RestTalk Machamp:
Machamp @Leftovers
Guts Ability
Adamant Nature; 252 HP/252 Sp.Def/4 Speed
~Rest
~Sleep Talk
~Bulk Up
~Cross Chop
The second one is not as common, as it's a little bit harder to pull off, and requires good scouting of the opponent's team. However, if used correctly, can rip through teams with Machamp's awesome Attack, and deceptively good defenses. Be warned this set is walled by Ghosts, so Bulk Up should be considered optional.