I'd agree with you if Arceus didn't count, especially how Rayquaza owns with all kinds of attacks. But yeah, Mewtwo would probably win hands down with Special Attacks (Because of it's amazing Sp. Attack) and it could defeat anything with probably just one Psychic, although, it can increase it's Defenses with Amnesia. In fact, Mewtwo can also learn Recover so it will make itself unbeatable against Rayquaza, which, if used in games, works for 20 times! If this happened in the games, Mewtwo would have pretty much destroyed ANY legendary without even lowering the PP of Recover to 0! Also, it has the ability Pressure, which Arceus would NEVER have, which makes the opponent twice as easy. Like stated, it could also block attacks, which makes the other Legendaries a little weak against Mewtwo, and of course, Mewtwo would win in the end.