Mewtwo's best PvE moveset is Psychocut/shadow ball in my oppintion based on practice, spreadsheets list Confusion, but psycho cut's much more dodge friendly to. (confusion/psychic is its best nonlegacy set for psychic specialist) and if you have a lot of CTM's, can be an (expensive) tm'able protean If you dont have the other attackers of those types
For PvP, Psycho cut and a combination of shadow ball, and icebeam/thunderbolt/possibly focus blast, but theres not much weak to fighting aside from tyranitar/meltetal (tyranitar in particular absolutely eats mewtwo because of B/C in my experience). This is mostly for masters, but in masters theres a lot of other pokemon that are fantastic, and mewtwo's big PvP weakness is it absolutely needs shields to be cleared out before it comes in late in the fight (generally your last pokemon), as most things in masters resist psychic type and its damage is almost exclusively charge moves, and being paired with a shield breaker early on is a big tell mewtwo's in the last slot. I wouldn't recommend mewtwo for PvP first, it is good, but there are others that i've found better/more reliable and it's is very dependent on charge moves, making it very easy to dismantle. Having used one, when mewtwo has those match ups to win, it wins by a long shot (basically its very hit or miss; isnt easy to recover from a miss, and the opponent can do almost nothing if you get the hit) but if mewtwo is a staple of your team, its moveset will be dictated by the other two pokemon (you wouldn't want ice beam if you have a PS/Av mamoswine on the team for example) For PvE, i'd recommend psychic on mewtwo as a second move (even with PS as the fast move its still best psychic attacker, and in practice i've preferred the dodge friendly nature rather than FTM'ing it ever time I want to raid for a near negligible DPS gain) as the other moves it has, while great at those types (aside from fire and debatably fighting) are done better by nonlegendaries
Giratina I haven't heard much about in masters, as in there there are other dragons (legacy dragonite and latios), its place to shine in ultra. Dragon breath or shadow claw /Dragon claw and ancient power are more or less the best set for it (shadow sneak also does well, but ancient power is somewhat preferred with outrage snorlax these days) but a second move isnt overly critical in my oppinion. I'd say go with giratina for the second move first, it is a much more reliable pokemon in its PvP league. If you have the luxury of enough candy for mewtwo, i'd add the second move to it just to have incase you need it/they rebalance PvP again
Sorry about the long response
