Is there any Pokemon more powerful than Mewtwo?

Actually, no, Arceus is NOT stronger than Mewtwo. In the game, he may have higher stats, that's true, but Mewtwo's ability to Mega Evolve shoots his stats up even higher than Arceus. It's been emphasized and even outright stated in the games, anime, manga, and interviews that Mewtwo is the strongest. The fact that Arceus is the Pokemon god is irrelevant, but most people believe he's the strongest simply for that reason or because of his higher stats to Mewtwo's base form. But stats don't play a role in the anime or manga, which is much more realistic.

Also very common theme in anime is that humans somehow surpass or defeat God in some way, either through their own strength, a special power, a mecha, etc. in the case of Pokemon, it's Mewtwo. Besides, even though Arceus was introduced much later, it was still repeatedly stated that Mewtwo is the strongest. Arceus was always there for thousands, even millions of years, and the characters knew about him. Just we as an audience did not. You can't have everyone in the series (including the omnipotent narrator) say that Mewtwo is the strongest, then introduce Arceus and say "Oh no, Arceus is actually the strongest and was there all along, we just said Mewtwo was the strongest because you didn't know about him". In fact, one of the reasons why they gave Mewtwo a Mega Evolution and not Arceus, so there's no doubt that he's superior.