Right now the best moveset is hands down PC/SB because that makes Mewtwo a great counter to Lugia. I think it's also overall the best moveset to have, as it's not that much behind C/SB in neutral damage but also makes M2 a great counter to psychics.
I have never had mine with anything other than SB because it also performs matchups where Psychic would be good (ok against Machamp, better against Gengar) and I have a Machamp army that makes FB Mewtwo completely unnecessary. But I have changed the fast move a few times depending on the situation.