Psycho cut is mewtwo's best fast move in all but one case these days. (that case being mewtwo fighting pokemon weak to psychic)
A bit more information on what other pokemon you have would be useful, like other legendaries. Mewtwo, with the other moves is very strong, but there are many nonlegendary substitutes that can do the same type offense as mewtwo's other moves (aside from psychic and shadow ball) and do those better/cheaper.
Mewtwo from what ive seen in PvP is very hit or miss, its strong, but it often doesnt have the bulk to deal with things like B/C tyranitar, which is very common and extremely dependent on having all the shields gone to be able to do things effectively. Shadow ball's its main move ive seen, after that maybe ice beam, but that is mostly for legacy dragonite. If you plan to add one, focus blast or ice beam are the best to add