Nidoking

Nidoking suffers from middling stats and an unfortunate typing. In the main series games, Nidoking was a jack of all offensive trades, but he misses his trademark massive movepool in GO. Poison and Ground are poor attacking types, lacking relevant type coverage or powerful moves, and Nidoking’s Atk stat is disappointing to boot.

On defense, Nidoking is frail and weak to common attacking types. Its only multi-bar move is Earth Power, but it's easy to dodge, making it a mixed-bag. Nidoking is not a good defender.

Best PvE Offensive Moveset

• Poison Jab is slightly superior to Iron Tail for STAB and type effectiveness.

• Fury Cutter has a fast cooldown and high EPS that performs surprisingly well despite lacking STAB.

• Earthpower is Nidoking's best charged move, though it lacks general synergy with any of its fast moves.

• Sludge Wave is only one of Nidoking’s charge moves that can pair typing and STAB with one of Nidoking's Quick Moves. It has limited but unique type coverage with STAB.

• Earthquake is basically a worse Earthpower.

• Megahorn is useless on Nidoking.

• Sand Tomb is meant for PvP.

Best PvE Defensive Moveset

• Iron Tail is superior for raw damage output against Nidoking’s numerous type counters.

• Poison Jab is decent on defense, but Fury Cutter is completely ineffective.

• Sludge Wave has a combination of high power and not being utterly trivial to dodge. However, it's one-bar, meaning it won't come out very often.

• Earth Power deals good damage as a 2-bar move, but is easy to dodge. 

• Sand Tomb as a 3-bar move is great for defense, and can be used frequently. 

• Megahorn is weak, but it has the earliest damage window Nidoking’s charge moves.

• Earthquake is outclassed on defense.

Access to Fury Cutter lets Nidoking stand out from other Pokemon, most notably Nidoqueen. While its damage output is somewhat laughable, its faster energy gains enable Nidoking to access its Charge Moves sooner. Poison Jab is also viable due to its significantly stronger STAB damage output, but going for this option would make Nidoqueen seem more appealing due to her greater bulk. 

Earth Power is the generally preferred ground-type attack due to its faster charge time, though Earthquake offers more raw power and power per energy spent. Megahorn provides solid power and aforementioned counter-coverage against Psychic types, making it the preferred second charged-move. Sand Tomb is recommended for its cheap energy cost, enabling Nidoking to bait shields more easily, as well as applying a guaranteed debuff on the opponent.