Darth Bane's power - Official Statement
If Failkorion supporters accept singular statements like the Shit Emperor being the most powerful Force user up to his time as definitive (the only other similar statements are the in-universe statements of historians, and if one accepts those then they must also accept this), then they must also accept that Bane > all those who wielded Sith power before him. Given how you don't need to be Sith to use Sith powers, Failkorion qualifies and is thus canonically inferior to Bane.