I will make a list of how some pokemon deals with Aegislash:
Mega Charizard X: One of the few pokemon who can OHKO Aegislash in Shield Forme. However, Shadow Ball can 2HKOes back.
Landorus: One of the few pokemon who can OHKO Aegislash with Life Orb Earth power. However, Shadow Ball 2Hkoes back.
Mega Mawile: To win, it has to be sure that Aegislash is in Shield Forme, or otherwise it will be very difficult. Sucker Punch only 2HKOes, fire Fang barely 2HKoes, and both Shadow Ball and Flash Cannon 2Hkoes back.
Thundurus: Thunderbolt only causes a 3HKO, while Shadow Ball 2HKOes.
Azumarill This specis is full of contact moves, none of them are super effective. CB Waterfall has a chance of 2HKO but Shadow Ball and Flash Cannon also have a chance of 2HKOes back. Fears King's Shield.
Bisharp One of the few pokemon who can deal realiably with Bisharp thanks to Defiant neutering King Shield drop. However, Sacedd Sword OHKoes with no Attack investment.
Clefable Steel Stab demolishes Clefable. Shadow Ball clearly 3HKO (and can 2HKO with SR if Unaware). What Clefable can do back? Not much, unless got a few Calm mind plus a Fire move.
Excadrill Not so easily. Sacred Sowrd + Shadow Sneak will OHKO. Shadow Ball + Shadow Sneak could OHKO Excadrill. True, it has EQ but Excadrill needs LO to be able to OHKO Aegislash.
Garchomp Depending of the set EQ could be able to OHKO, but it's not a guarantee in many of Garchomp sets. With no defensive investment, Shadow Ball and Flash Cannon can 2HKO Garchomp.
Greninja With Dark Pulse it can 2HKo Aegislash However, it fears a possible OHKO from Sacred Sword. Greninja si surpriseingly not 2HKoed by any of his STAb moves if Greninja has the Water/Dark type. Hydro Pumo has a chance of 2HKO.
Mega Gyarados One of the few pokemon who can use Earthquake to clearly 2Hkoes Aegislash. Also, only Sacred Sowrd can 2HKO Mega Gyarados. However, it fears the issue of using King Shield if he chose Waterfall (who does 3HKO it at +0)
Keldeo Hydro Pump is a guarantee 2HKO (if it hits) while Shadow Ball 2Hkoes back. However, Scald doesnt guarantee a 2HKO. Keldoe has a huge problem: it's usually a Choice pokemon and fears getting locked into Secret Sword.
Mega Pinsir Requires Earthquake to 2Hko Aegislash. Return is only able to 4HKO Aegislash. However, Aegislash can use either of his STAb to clearly 2HKO Mega Pinsir.
Mega Scizor Knock Off has a very tiny chance of 2HKO while Aegislash can 2HKo the Mega form with Shaow Ball. However, every attack that Scizor uses is affected by King Shield, with is also triggered if he uses U-turn. And ah, U-turn does ridiculous damage to Aegislash (maximum of )%). Bullet Punch does a maximum of 30% while at +2.
Talonflame One of the few pokemon who can Aegislash with Flare Blitz (with Adamant + CB). However, Shadow Ball does a clear 2HKO. Also, Talonflame absolutely requires Stealth Rock out of the field.
Mega Tyranitar The good thing is that Tyranitar is only 2Hko by Flash Cannon and Sacred Sowrd (if uninvested in attack). Crunch and Earthquake 2HKo Aegislash. However, Crunch triggers King's Shield.
Tyranitar Here is worse because exist chances that EQ is a possible 2HKO. Also Sacred Sword can OHKo many variants of Tyranitar.
Mega Venusaur. 40% is toomuch for Mega Venusaur because mega Venusaur has no ways of delaing with it, even with HP Fire or Earthquake, who can't 2HKO it (and the former can't 3HKO with max SpA) Giga Drain is useless against Aegislash. the only chances to win s through Sleep Powder + Leech Seed.
Mega Charizard Y One of the few pokemon who can OHkoA egislash It doesn't fear Kingp0s HSield siode effects and any Firee moves OHKOes Aegislash in sun. Shadow Ball has a chance of 2HKo, which means it¡'s not a counter.
Dragonite The most common set is the CB set. That set does a 80-90% to aegislash with EQ.
Ferrothorn Shadow Ball can 3HKO Ferrothorn. While Ferrothorn can't do anything back expect sedding it.
Gengar An fair match? Absolutely not. Even though the two are weak to Shadow Ball, Gengar does to Aegislash a maximum of 67,2% with Back Sludge while Aegislash does a maximum of 180% (guaranteeeed OHKO) with the Same Shadow Ball. Also two of the moves of Gengar is completely ineffective against it. Requires an Specs to have a chance of OHKOing Aegislash.
Heatran it does do well, but not so excellently. lava Plume can 2HKO Aeghislash but Sacred Sword can 2Hko Heatran. However, it loses to physical Aegislash if he uses a Sdef set.
Hippowdon Requires a special set if he wants to deal twith Aegislash because otherwise Shadow Ball can 2HKO. Exist an option to guarantee defeating Aegislash: putting an Speed reducing nature and put the IVs to 25.
Landorus-T. However, having a wall trying to underspped to be able to defeat a threat is considered overcentralization.
Mandibuzz One fo the few counters available. Or sort of counter. Foul Play is unaffrfected by King Shield and is able to 2HKO Shield Aegislash. Only Flash Cannon is able to 3HKO iMandibuzz. However, there's the Sd set.
Latias It does horribly. The onyl good thing is thanks to her special bulk, Shadow Ball is not guarantee to OHKo.
Latios Same than Latias, with the bonus of Shadow Ball almsot always OHKoing Latios.
Rotom-W. Aegislash wins because his damage output is rather weak. Hydro Pump only 4hkoes while Shadow Ball clearly 3HKoes.
Terrakion.Only EQ can do something to Aegislas. However, everything else is really bad: all of tthe res tof mvoe sare really inefective (or inmune), Aegislash can use any move to deal with it, Flash Cannoin has a chance of OHKoing Terrakion, and has a tendency of usign Choice Items.
Breloom Aegislash hard counters it. His fraility causes that a neutrla move can OHko Breloom.
Chansey Chansey can't do anything to aegiuslash If it faces a physical Aegislash, bye bye.
Diuggersby One of the few species whio is able to OHko Aegislash with EQ. However, Flash Cannon can 2HKO easily. One of the few good choices against it.
Gliscor Requires a Sdef to be able to defeat it and guarantee to avoid a 2HKO. Earthquake is not guarantee to 2HKoAegislash unless it's invested.
Kyurem-B aegislash is able to counter it rather easily.
Mamoswine It can 2HKo with Earthquake. However, Aegislash can OHko Mamo with Flash Cannon,or with the combo of Shadow Ball + Shaow Sneak.
manaphy Without tail Glow , Surf is not able to 3HKo this thing. Also, Shadow Ball has a chance of 2HKO it.
Scolipede Without EQ, Aegislash counters it, specially if either of those STAB 2HKoes Scolipede.
Skarmory Shadow Ball destroy Skarmory. Brave Bird is very weak.
I think that Aegislash has very few counters, a lot of pokemon who can walls and can defeat it.However, the aspect who mades it truly broken is the effect of King Shield who causes that if you touch Aegislash during Kingps Shield, his attack go down to -2, which is ridiculous when Aegislash has only 6 types who can neutral or SE damage
I think that Aegislash deserves to be banned because his extensive movepool (who unlike other species, has very different checks), his raw stats, his ability to ctake hits at 150 Defenses while always attacking with 150 offenses, hus ability is so uniqaue that even Ditto can't copy it, having a lot of very effective sets, fitting on every team and specially, the ability of King's Hield causing too much 50/50, speciallying when a wrong decision can cost the oponent to have a physical attacker at -2 (half of the original stat), which is huge.
Let me fix some things:
Mega Charizard X: Still a check.
Mega Mawile: What? Mawile can just spam Sucker Punch over and over, the attack reduction doesn't work if Aegis uses KS because it's not a damage move.
Azumarill: Belly Drum Azumarill can still set up on its face.
Clefable: Calm Mind + Fire move is the standard Clefable set. And if Aegis has any other coverage move that is not Iron Head, it is walled.
Garchomp: Garchomp is still faster and attacks first. It's a check.
Mega Gyarados: Then don't use Waterfall? Why are you posting calcs for moves that will never be used?
Mega Scizor: Scizor can keep spamming Swords Dance + Roost and use Aegis as set up fodder, though. Aegislash has to guess the exact turn Scizor is going to stop healing itself and use Knock Off, otherwise it's attacked in blade forme.
Mega Tyranitar: Don't use Crunch then, use Earthquake. Again, don't post calcs for sub-optimal moves.
Mega Venusaur: Then use Sleep Power or Leech Seed, and then spam Synthesis.
Mega Charizard Y: Still a check.
Ferrothorn: Which is enough if Ferrothorn is using a specially defensive set.
Hippowdon: the specially defensive set is the standard one, though.
Landorus-T: I think this one is missing from your post. Aegislash has the advantage if it's a scarfed set because of KS, otherwise Lando wins.
Rotom-W: Special Defense Rotom can burn and run, though.
Terrakion: Then don't use everything else, use EQ.
Breloom: Not that hard, Breloom can still put it to sleep for free, then switch into something else, or stay in and try to weaken it. And a sleeping Aegislash is in danger of taking massive damage before it wakes up if it's a LO Breloom. It is also forced to guess when it is gonna wake up, otherwise it risks being attacked in blade forme. The match-up is largely dictated by hax, it's not a hard counter.
Chansey: Most Aegislash sets can't do anything to Chansey, either. And she can still heal herself and her teamates.
Diggersby: Still a check. And almost a counter if you manage to switch into a ghost move.
Kyurem-B: No, no, no. It can survive a single hit from any move. It can 2HKO or 3HKO (depending on the sets) with Earth Power. And Sub Kyurem-B can force aegis into mindgames where the dragon has the advantage.
Mamoswine: The offensive set gets a guaranteed OHKO though. Check.
In the end, Aegislash has quite a lot of checks.
There are quite a few flaws in your post:
Doesnt that imply that it was a pro-ban argument, if its best checks 2hko'd? Even if they are fast, how are you planning them to get in, if they are 2hko'd, they can only switch in once, and with possible hazard damage/chip damage they could be ko'd with shadow ball + shadow sneak.
One of the main proban arguments is that it has NO 100% counters, just a bunch of checks, not many pokes want to take a shadow ball/sacred sword and the ones that can be beaten by sub toxic. Im not saying it has all those sets at once, but it has the possibility to run a set which can beat your 'counter'. If 'he'll leave a huge dent', you dont really counter it too well.
Not really, most pokes can run multiple sets, dnite, xzard etc, but aegislash runs several sets very well to a point that it is arguably broken. If you look at lucario, it could run np or sd, but that wasnt broken because of versatility, it was banned due to the opp needing 3 counters to check 1 set. Charizard is very versatile but not op, it can run y-wallbreaker, ddxzard or tankxard.
I also havent yet mentioned kings shield which imo pushes it over the broken line. It can be played around but it is still there - a wrong prediction could be devastating on either side, no poke should be able to force this which makes aegi uncompetitive.
The list of pokemon in this meta who have zero, or near zero, counters is pretty high though. I think it has already been estabilished that this gen is not a gen of counters. It is a gen of checks. Having zero counters would be an issue if this was a sweeper like Mega Luke, but in Aegislash's case, it just makes the game more prediction based.