Nihilego

Nihilego paves the way for the introduction of Ultra Beasts into Pokemon Go, with huge stats and an interesting typing. It is the best Poison attacker in the game, but this utility is rather limited and mayb be outclassed by other types. For areas with frequent cloudy weather (or when a Mega Evolved Poison-type Pokemon is in play), Nihilego can be one of the top options for taking down Fairy-type bosses.

Best PvE Offensive Moveset

• Poison Jab is Nihilego's best Fast Attack. Acid is very slightly worse.
• Pound lacks STAB and can't be Super Effective against anything. 
• Sludge Bomb is the best Charge Move for offense, with more flexibility as a 2-bar move.
• Gunk Shot is less consistent as a 1-bar move.
• Rock Slide is better for Rock-type Pokemon.
• Power Gem sucks. 

Poison Jab is a solid Fast Attack, providing above average damage and energy generation. Acid and Pound are both very poor moves.

Rock Slide is Nihilego's best Charged Attack, dealing good damage and having a moderately cheap energy cost. Sludge Bomb costs a moderate amount of energy, and benefits from STAB to deal respectable damage. Gunk Shot's high cost might seem like a strange move to consider, but Nihilego can often already perform its job simply with Poison Jab and Rock Slide. As such, Gunk Shot can be used simply as a surprise burst damage option. Power Gem has no real use because it has both lower DPE and higher energy cost than Rock Slide.