AM Demon's power that Asta's been using is but a small fraction of it's power. Asta never even used it's
true power before Dante fight. The movement AM Demon shared it's power with Asta he gained enough power to defeat this
This is Lucifero's host utilizing his maximum potential. From what we've seen, demon hosts can access their demon powers better than Asta and they can draw more power from their respective demons to augment their own. The Dark Triad demons can even take over their hosts at will unlike Asta's demon and yet all that power Dante mustered was meaningless against AM Demon's power. If Asta, who is technically not even a host, can do this to the alleged strongest Demon's host then imagine what AM demon can do with a host; imagine what it can do with it's original body.
Yes, the fact that he's the source of the MC's power is also a part of the reason why I think AM Demon might be the strongest. The anti-magic in itself is more than qualified enough to be the most broken ability in a world where everything runs with magic. To top it, AM Demon seems to be particularly overpowered even for a demon. A high ranking Demon like Zagred was scared shitless upon looking at Asta's
demon. Look at that expression, He must be something else if he can scare a high ranking Demon with his presence alone. Nacht outright confirms that anti-magic has the potential to make Asta the
ultimate worrier. I think that says a lot about AM Demons prowess. The only thing that's restraining Asta from human is his own lack of skill and strength to wield his demons true power.