They can be a pain to deal with...
Azelf, Starmie, and Togekiss are all great counters to machamp, because they outspeed and resist his feared move dynamic punch. You just slap a psychic type move, (or flying for togekiss) and shazam! You have a counter to machamp!
They can switch into DynamicPunch with impunity, and ghost types like Spiritomb, Sableye and Mismagius can proceed to cripper with Will-o-wisp. Other ghost types can switch in and deal serious damage.
Faster flying types
Probably work better if you don't try tank a DynamicPunch with them, but Flying Types can come in and destroy Machamp. Air Slash Togekiss (Lol flinch hax). Brave Bird Talonflame. You name it.
Faster Psychic types
They can deal serious damage, though you do need to remember that they may not OHKO due to Assault Vest and since most psychic are fairly physically defensively weak, Knock Off will be able to deal major damage or kill.
Fairy types like Sylveon can deal quite a bit to Machamp, or kill it outright with fairy STAB. They also resist Dynamic Punch, and even though their physical bulk is normally inferior to their special bulk, they can normally take a hit or two and respond in kind to Machamp