Good thread idea here, danteiscool.
Acnologia gets pretty far. He can easily take down the Weird Fangs, Dawn Roar, and the lower level Holy Knights (Twigo and Jericho). He pretty much just Dragon Roars them before they can do anything.
While we still haven't seen the full power of most of the Sins, I think they could still give him trouble. Diane's attacks with Gideon are mountain level in power and King's attacks with Chastiefol are pretty cazy, especially with its homing capability, the ability to multiply into smaller kunai-like weapons for barrage attacks, energy blasts, and petrification.
Meliodas and Ban have some good showings, even without their Sacred Treasures, such as splitting a mountain with a freaking twig and Ban punching Hendrickson down a good many feet below the surface. Geera, Hauser, Gilthunder, and Griamor are pretty tough, too.
Further more, you also have Helbram's Link as well as the stronger Holy Knights like Dreyfus who could stop Diane's Ground Gradius with his sword and Hendrickson is shaping up to be a real monster.
I wonder if Merlin and Oslo could BFR him?