What makes a moveset good is the situation it is curated towards. Movesets that value the charged attack will be different from movesets that value the maximum damage output.
Pokemon GO: Best moves for Grumpig
The first thing to keep in mind before even using a certain Pokemon is what type it is and what its weaknesses are. Grumpig is a pure Psychic-type which means it is weak to Bug, Dark, and Ghost-type attacks. Pokemon like Gengar or Pinsir will deal with Grumpig very easily. Grumping also has the advantage over Fighting and Poison-type Pokemon. Pokemon like Toxicroak and Muk can easily be taken care of by Grumpig.
The first fast attack Grumpig has access to is Charge Beam. Charge Beam is an Electric-type attack with a flat damage rate of 8. In Pokemon GO, Charge Beam is a slow attack that has a damage output of 7.3 health per second. However, Charge Beam generates a lot of energy with every use.
Grumpig's next fast attack is Extrasensory. This move is generally recommended in all builds that do not prioritize the charged attack as it deals the most damage and is also boosted by the same type attack boost.
For charged attacks, the best option is Psychic. Psychic is Grumpig's most powerful charged attack with a base damage of 90. Psychic also has a damage per second output of 38.6 hit points per second which makes it the strongest option for Grumpig's charged attack in Pokemon GO.
Aside from Psychic, Grumpig's next best option in Pokemon GO is Shadow Ball. This is a great coverage option as Ghost-type moves are super effective against Psychic and Ghost-type Pokemon, giving Grumpig a counter for any Ghost-type Pokemon that would switch in or against another Psychic-type. However, Shadow Ball should be run with Extrasensory in order to have some good damage output with a fast attack.
In summary, Grumpig has a lot of options in terms of sets to run in Pokemon GO. However, the best set for damage output uses Extrasensory and Psychic, and the best set for a more defensive playstyle is Extrasensory and Shadow Ball.