Mewtwo is always in the Psychic slot. One of the most iconic Pokemon in the TCG โ here's which sets have the best Mewtwo pulls for breaks.
Find a Mewtwo Break โPokemon GO (SWSH10.5): The best Mewtwo break set โ Mewtwo VSTAR Alt Art ($40โ90) and Mewtwo V Alt Art ($20โ45) both in Psychic. A rare case of two Mewtwo Alt Arts in the same set.
Scarlet & Violet 151 (SV3.5): The nostalgia set featuring OG Kanto Pokemon. Mewtwo ex SIR ($15โ35) is a solid pull. Note that 151 also has Gardevoir ex SIR in Psychic as a secondary hit (since Gardevoir was formerly Fairy, now Psychic in SV era).
Chilling Reign (SWSH6): Calyrex Shadow VMAX Alt Art (Psychic, $80โ200+) is S-tier here โ while not Mewtwo specifically, it makes Psychic the best slot in the set.
Always Psychic. Mewtwo is a Psychic-type in the TCG and always goes to the Psychic slot in breaks. This has never changed across any set era.