The best opponent would be Tyrannitar:
https://rankedboost.com/pokemon-go/mewtwo/
https://pokemongohub.net/gym-raid-u...mewtwo-raid-boss/#1505129569685-35b2c288-9728
https://rankedboost.com/pokemon-go/mewtwo/
https://pokemongohub.net/gym-raid-u...mewtwo-raid-boss/#1505129569685-35b2c288-9728