Nothing. Switches. In. Everything's weak to Aegislash
I'm assuming we are talking about standard Aegi with Sacred Sword and Shadow Ball Linked, as well as KS and Shadow Sneak.
So here is a nice list of "Nothing".
Calm Mind Meloetta
4 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 188 Def Meloetta: 92-109 (22.7 - 26.9%) -- possible 5HKO after Leftovers recovery
Aegislash-Blade Shadow Ball vs. Meloetta: 0-0 (0 - 0%) -- aim for the horn next time
Meloetta runs Shadow Ball too, and can run CM + Attack.
Doesn't fear KS, SS, or FC
Eviolite Murkrow
4 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 0 Def Eviolite Murkrow: 121-143 (37.3 - 44.1%) -- guaranteed 3HKO
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 252+ SpD Eviolite Murkrow: 63-74 (19.4 - 22.8%) -- possible 5HKO
This thing has actually proved to be really strong with Prankster Feather Dance + Roost linked.
Doesn't Fear KS, SS or FC
SpD Mega Sableye
Aegislash-Blade Sacred Sword vs. Mega Sableye: 0-0 (0 - 0%) -- aim for the horn next time
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 252+ SpD Mega Sableye: 105-124 (34.5 - 40.7%) -- guaranteed 3HKO
This generally doesn't run CM. Between Will-O-Wisp and Foul play physical attackers aren't that scary, plus you out slow and OHKO Aegi.
Doesn't fear KS, SS or FC
Mandibuzz
4 Atk Aegislash-Blade Sacred Sword vs. 248 HP / 136+ Def Mandibuzz: 71-84 (16.7 - 19.8%) -- possible 7HKO after Leftovers recovery
252+ SpA Aegislash-Blade Shadow Ball vs. 248 HP / 108 SpD Mandibuzz: 75-88 (17.7 - 20.8%) -- possible 7HKO after Leftovers recovery
Lol as if I need to post calcs, we all remember XY.
Doesn't fear KS, SS or FC
Drapion
4 Atk Aegislash-Blade Sacred Sword vs. 248 HP / 0 Def Drapion: 85-101 (24.7 - 29.4%) -- possible 5HKO after Black Sludge recovery
252+ SpA Aegislash-Blade Shadow Ball vs. 248 HP / 216+ SpD Drapion: 72-84 (20.9 - 24.4%) -- possible 6HKO after Black Sludge recovery
Wouldn't recommend cause it can only come in once, can't do much once in (maybe T-Spikes), and is threatened by KS. But if you're anything like my older brother Drapion is your favorite pokemon, so if you wanna use it, just know that Aegislash can't 2HKO which is nice.
Porygon 2
4 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 4 Def Eviolite Porygon2: 136-160 (36.3 - 42.7%) -- guaranteed 3HKO
Aegislash-Blade Shadow Ball vs. Porygon2: 0-0 (0 - 0%) -- aim for the horn next time
Regardless of whether you are SpD or Def, neither Flash Cannon or Sacred Sword can 2HKO, muh Eviolite bulk.
Doesn't fear KS, SS or FC.
Chansey
4 Atk Aegislash-Blade Sacred Sword vs. 4 HP / 252+ Def Eviolite Chansey: 246-290 (38.3 - 45.1%) -- guaranteed 3HKO
Aegislash-Blade Shadow Ball vs. Chansey: 0-0 (0 - 0%) -- aim for the horn next time
Chansey is actually really bad in this meta because the power spike means most can break it and it is way too passive giving free set up to everything, but it is funny that it checks Aegi.
Doesn't fear KS, SS or FC.
Miltank and Lickilicky are also mono normals that can stomach Aegislash hits, I haven't used them though so idk if they have any good links.
Bulk Up Braviary
4 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 0 Def Braviary: 117-138 (28.9 - 34.1%) -- 99.4% chance to 4HKO after Leftovers recovery
Aegislash-Blade Shadow Ball vs. Braviary: 0-0 (0 - 0%) -- aim for the horn next time
Bulk Up + Roost or Bulk Up + Attack could be solid
Doesn't fear SS or FC. KS is not that scary cause you boost anyway, so you don't go into negative attack, and either Roost or Bulk Up would be unlinked to not attack while Aegi uses KS.
SpD Golbat
4 Atk Aegislash-Blade Sacred Sword vs. 248 HP / 0 Def Eviolite Golbat: 20-24 (5.6 - 6.7%) -- possibly the worst move ever
252+ SpA Aegislash-Blade Shadow Ball vs. 248 HP / 252+ SpD Eviolite Golbat: 93-109 (26.3 - 30.8%) -- guaranteed 4HKO
Stomachs any hit from Aegi with absolute ease, not sure if it has any good links though cause I've never used it.
Doesn't fear SS or FC, has plenty of ways around KS.
SpD Mega Venusaur
4 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 0 Def Mega Venusaur: 39-46 (10.7 - 12.6%) -- possible 8HKO
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 240+ SpD Mega Venusaur: 102-121 (28 - 33.2%) -- guaranteed 4HKO
Sub-Seed maybe?
Doesn't fear KS, SS or FC.
AV Amoonguss
4 Atk Aegislash-Blade Sacred Sword vs. 248 HP / 8 Def Amoonguss: 61-72 (14.1 - 16.7%) -- possible 6HKO
252+ SpA Aegislash-Blade Shadow Ball vs. 248 HP / 252+ SpD Assault Vest Amoonguss: 90-106 (20.8 - 24.5%) -- guaranteed 5HKO
Probably the best Aegi Counter, as not only can it stomach all its hits, it out slows and OHKOs with Foul Play, plus it checks a bucket load of other top threats in the tier.
Doesn't fear KS, SS or FC
Don't wanna waste time working out the perfect EVs, but if AV Tornadus-T, spreads its SpA investment between HP and SpD it can survive a couple hits from Aegi, while still having great speed and utility in the Knock-Off + U-Turn link.
SpD Dragonite
4 Atk Aegislash-Blade Sacred Sword vs. 216 HP / 0 Def Multiscale Dragonite: 24-28 (6.3 - 7.4%) -- possibly the worst move ever
252+ SpA Aegislash-Blade Shadow Ball vs. 216 HP / 228 SpD Multiscale Dragonite: 64-76 (16.9 - 20.1%) -- possible 7HKO after Leftovers recovery
I VSed one of these and it is actually REALLY strong, and there is less opportunity cost in running it now that DD + Sky Drop and Protect + Extreme Speed are banned. A Sub + Roost Link with Multiscale is really hard to break and can then proceed to T-Wave and Dragon Tail from behind sub.
Doesn't fear KS, SS or FC
SpD Eviolite Scyther
4 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 4 Def Eviolite Scyther: 18-22 (5.2 - 6.3%) -- possibly the worst move ever
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 252 SpD Eviolite Scyther: 97-115 (28.1 - 33.4%) -- 0.1% chance to 3HKO
This is just another random thing like Miltank which I listed cause it can take on Aegi and has reliable recovery, but I'm not sure if it has a niche out side of that so yeah. But if you find one it could be cool.
Doesn't fear SS or FC, has ways around KS.
Bulky Volcarona
4 Atk Aegislash-Blade Sacred Sword vs. 248 HP / 136+ Def Volcarona: 49-58 (13.1 - 15.5%) -- possibly the worst move ever
252+ SpA Aegislash-Blade Shadow Ball vs. 248 HP / 0 SpD Volcarona: 153-181 (41 - 48.5%) -- guaranteed 3HKO after Leftovers recovery
252+ SpA Aegislash-Blade Shadow Ball vs. +1 248 HP / 0 SpD Volcarona: 102-121 (27.3 - 32.4%) -- 64.1% chance to 4HKO after Leftovers recovery
This means that Quiver Dance + Roost linked bulky Volc can switch in on Aegi, set up in its face and OHKO with Fiery Dance.
Doesn't fear KS, SS or FC
I bolded the things which I think are most spashable, but any of these things should be viable on select teams.
EDIT: Hopefully this will help more with team building as well as proving a point.