You honestly don’t really need one as he’s not vital in the current meta. He does make a great generalist, but so do dragonite and ttar. The only place where I can see him making a huge impact is as a machamp counter for those who solo threes, but this can also be accomplished easily without a mewtwo.
That being said, I maxed mine out. He’s my favorite Pokémon, and I find him very fun to use.