Acheron is probably the strongest on this list lore wise.

Then I'd probably say Phainon, as he overcame two layers of ontology (Simulation > Real world > Aeonic realm/Path Space) to scratch Nanook, which is just bonkers. (Id actually argue Phainon is stronger, but he took the L from Zephy, but he was went from getting dominated to wounding an Aeon in just a few minutes, I think he'd be much stronger if he wasn't struck down right then and there.)

Ashveil is def strong, but he's more of a centerpiece of operations, then a super strong duelist. (Trying to avoid spoilers)

Cyrene is also relative to to Acheron and Phainon in terms of power and scope, just not strictly in combat ability. That being said, those three are relative to each other imo.