What this stems from is what the official canon said to being with. Mewtwo was called the most powerful Pokemon over and over again. It was designed to be a "final boss." An ultimate Pokemon to complete the original Pokedex.
But the problem is that they haven't called him that since Arceus was introduced. BUT that doesn't mean Arceus is now most powerful; they haven't called Arceus the most powerful either. But that doesn't mean they were hailing Mewtwo as the ultimate Pokemon and suddenly stopped when Arceus came out. They pretty much ignored Mewtwo since Gen 3. Even when FRLG came out Mewtwo was ignored. So we don't know if Mewtwo is still the canon "most powerful Pokemon."
Further confusing the issue is the fact that western religion is our frame of reference when we hear about a 'god' Pokemon. People tend to equate Arceus with the Judeo-Christian God. It's not. Arceus draws heavily from Shinto and Taoism; the "egg" origin myth is an exact copy of their origin myth. Arceus is not an almighty, infallible God. It's extremely powerful, but not omnipotent. Heck in the movie we even see Arceus on the verge of death being rescuing by a lowly human. That's not a western image of God.
My point in all this is that it's entirely conceivable that humans could create a Pokemon more powerful than the creator Pokemon. Arceus (allegedly) creating the world does not automatically mean it's the most powerful Pokemon ever.
Also, BST is no the end-all indicator of power. Otherwise Dugtrio wouldn't be four tiers above Regigigas.
But the problem is that they haven't called him that since Arceus was introduced. BUT that doesn't mean Arceus is now most powerful; they haven't called Arceus the most powerful either. But that doesn't mean they were hailing Mewtwo as the ultimate Pokemon and suddenly stopped when Arceus came out. They pretty much ignored Mewtwo since Gen 3. Even when FRLG came out Mewtwo was ignored. So we don't know if Mewtwo is still the canon "most powerful Pokemon."
Further confusing the issue is the fact that western religion is our frame of reference when we hear about a 'god' Pokemon. People tend to equate Arceus with the Judeo-Christian God. It's not. Arceus draws heavily from Shinto and Taoism; the "egg" origin myth is an exact copy of their origin myth. Arceus is not an almighty, infallible God. It's extremely powerful, but not omnipotent. Heck in the movie we even see Arceus on the verge of death being rescuing by a lowly human. That's not a western image of God.
My point in all this is that it's entirely conceivable that humans could create a Pokemon more powerful than the creator Pokemon. Arceus (allegedly) creating the world does not automatically mean it's the most powerful Pokemon ever.
Also, BST is no the end-all indicator of power. Otherwise Dugtrio wouldn't be four tiers above Regigigas.