Metagross. Starting with raids, since thats kinda the baseline for great attackers. ITs one of the best neutral DPS attacker in a game, only thing’s faster are either glass cannons, rayquaza, gengar (set dependent) and ramprados or are shadow ball mewtwo for the most part are all great neutrally for raids. All have their specific raid wins and raid losses, and the more glassy options fall off when nothitting super effective or resisted damage is being taken.
The grunt battles kinda automatically kicks out Rayquaza, Gengar and rampardos, too glassy to really stand up to attack jacked up grunt battles while they already struggle in regular masters and ultra (All do poorly in GL, aside from metagross who itself isnt the greatest, but does have its niche, while Gengar’s outclasses by its prevo, the legendaries are unable to get low enough and Rampardos….theres just so many steels, fighting, grass, waters and grounds that are great intop of glass cannon stats, so because all are relatively not good, this matters little), while metagross’ steel subtyping provides more reliable resistance than mewtwo, so without knowing what the rocket stop is, metagross is your best option for reiability.
In PvP both metagross and mewtwo are heavy hitters in master league, good offensive stats and offensive moves, metagross possesses superiors resistances and bulk, while mewtwo’s got that swiss-army knife moveset which allow both to threaten some of the top of the masters tier.
Gym’s are where mewtwo sorta would drop off a cliff if only one mewtwo set was considered, that being shadow ball. Metagross kinda just bashes gyms in with meteor mash, considering two of the most common types of gym defenders are normal and fairy. If only shadow ball mewtwo was considered, this is where mewtwo would get ended just by normal’s resistance to ghost, but if considering multiple sets , metagross and mewtwo still tie one another because focus blast access allowing it to take normal down faster than metagross, while metagross takes fairies down faster.
So it’d say it’s have to be a tie between meteor mash metagross and Shadow ball Mewtwo, with metwo only able to stay in due to double moves. If only one charge move was considered for each pokemon Outside PvP, and a single charge/fast set compared across all formats, mewtwo would have lost in the gym match ups (as the set that gives mewtwo best neutral DPS and the set that gives it best gym coverage are different sets, because comparing every single mewtwo set to pokemon running for the most part only one moveset would be somewhat unfair if the comparison is only single movesets), but thats just my oppinion