mega-kanga has no counters. If kanga didnt have scrappy there would be a few but as far as i know nothing can switch in and always force it out. He is not the only pokemon in ou with no counters though.
Talonflame. Brave bird beats out sucker punch and fake out. EQ doesn't hit, and it can basically 1 hit with brave bird.
Just to add something:
It's a 2HKO with a boosted Brave Bird. In addition to that, most MegaKhans only pack Power Up punch that can hit Talonflame. If the use EQ, it ain't working. If they use Fake Out, it ain't working because they'd need to switch out first/use it on the first turn. If they use Sucker Punch, +1 Brave Bird shuts it down (Sucker Punch FAILS if another Pokemon goes first, which is why Khan can WRECK Mega Mawile). So all they can do is use a 60BP move against Talonflame, which to my knowledge can not 1HKO Talonflame with full HP. Though I never let MegaKhans get past x2 or x3 for power ups, so, yeah. :<