Happiny's best moveset in Pokémon GO is Pound and Psychic, with 3.27 damage per second, and it can deal 50.0 damage before Happiny faints. This is true when battling in neutral weather.
All Happiny movesets ranked
Here is a complete list of Happiny movesets in Pokémon GO, ranked by their DPS and TDO. You can change the weather and the opposing Pokémon.