Galarian Yamask's best moveset in Pokémon GO is Astonish and Night Shade, with 5.24 damage per second, and it can deal 74.3 damage before Galarian Yamask faints. This is true when battling in neutral weather.
All Galarian Yamask movesets ranked
Here is a complete list of Galarian Yamask movesets in Pokémon GO, ranked by their DPS and TDO. You can change the weather and the opposing Pokémon.