Pretty sure SpD Gliscor carries Taunt, so between SubTect Aegi's temporary immunity can wear out pretty fast and gets prevented from Magnet Rising again, while Gliscor easily tanks and heals off Aegislash's hits. Aegi would also struggle trying not to expose itself in Blade Stance as it attacks into Protect or give Gliscor a free Sub as it King's Shield (I don't even think Magnet Rise Aegi has room for this); the matchup is very difficult for Aegi unless it gets lucky SpD drops from Shadow Ball or just carries HP Ice (which, tbh, might not be too farfetch'd considering its newfound advantage against Landorus and TankChomp). If Gliscor carries Knock Off, Aegi would most likely definitely lose as Gliscor can easily threaten a KO on Blade Stance Aegi even as it is floating. Meanwhile, Gliscor has a much easier time against other Aegi variants.
SpD Gliscor is by far the most consistent all-around Aegislash counter I've seen, but it's kind of the only one right now; other counters can get clipped by certain sets, or even beat only one Aegislash variant.
Amoongus is arguably even more reliable than Gliscor. It takes much less damage from Shadow Ball and has Regen, meaning it isn't basically forced to use a Recovery move each time it takes a Shadow Ball, hence give them a free switch into a Gliscor check, and can put the free turn to immediate use by using Spore for example. It also under-speeds it and KO's with Foul Play, can't be lured by HP-Ice, doesn't give a single shit about Sub Toxic sets either, and handles other prominent S-threats like Keldeo and Altaria.