There are plenty of Pokemon that top Mew in base stats and/or Mega/Primal form, but no Pokemon top Mega Mewtwo. Mew has 600 base power and Mewtwo has 680 base power, but Arceus has 720 base power. But if Mewtwo can Mega Evolve, it has 780 base power,which is 60% higher then Arceus' base power, making it the strongest Pokemon. Although, it shares its throne with Rayquaza, having 680 base power and 780 Mega Power.
So, if Mewtwo is not in Mega Form (both X and Y Mega),then Arceus is stronger. If Mewtwo is in Mega Form, Mewtwo is the most powerful, but Mew is not.
Arceus would be the most powerful if it could Primal or Mega Evolve, but since it can't, Mega Mewtwo, Primal Groudon, Primal Kyogre, Mega Rayquaza, and some others top Arceus.
So, Arceus has the best Base Stats, but Rayquaza and Mewtwo have the best Mega Stats.