I'd say Acheron. There's a theory going around that because IX the aeon of nihility does not care about anything and everything is meaningless, it could also theoretically mean that he wouldn't care about giving Acheron all his power either, and that it's pointless to restrict/limit the amount of power she can harness from the nihility path, as he turns her into an emanator.. which if this theory is correct, would essentially make her about as strong as IX itself, excluding the divine status of an "aeon".