>>20153661Any Garchomp (mega or not), Tyranitar (mega or not), Gliscor (if physical), Mega Pinsir, Azumarill, Conkeldur (if physical; if Assault Vested, then any Aegislash), Dragonite (Multiscale or not), Ferrothorn for stalling, Mega Gyarados (if special, or if physical without Sacred Sword), Heatran, Landorus T (if physical), Mega Mawile, Scizor (Mega or Not), Mega Venusaur, Mega Charizard, Mega Blastoise, Zapdos, Salamence, Volcarona (if special). And those are just things Aegislash would be meeting in the OU tier.
Out of OU, we still have: Mega Absol, Mega Aggron, Arcanine, Chandelure (in some cases, but my experiences with Chandelure vs Aegislash, my Chandelure wins [this is mainly if will-o-wisp Chandelure and physical aegislash, so you can disregard if this is too specific]), Darmanitan, Donphan (if physical), Goodra (if special), Haxorus, Hippowdon (if physical), Mega Houndoom, Krookodile (in most cases), Scrafty, Umbreon (stall it out).
So a ton of things in both OU and UU have the defensive and offensive capabilities to deal with most Aegislashes.