Nidoking's probably one of the best and easiest Pokemon to use In-Game. Whenever I use Nidioking In-Game in FR/LG, I tend to use this Moveset:
- Earthquake
- Brick Break
- Strength/ Thrash
- Megahorn (which is one of the few moves Nidoking learns naturally after you use the Moon Stone).
If you want, you could put Toxic on him or You can put the Thunderbolt/ Ice Beam Combo on him. However, Earthquake is a MUST regardless of what you choose to do. You can use Dig until you get Earthquake after you beat the Viridian City Gym.
Sludge Bomb is another option but you don't get it until the Post-Game when you beat Team Rocket in their Base on Five Island.
Regarding the Lapras vs Gyarados issue, it depends on what you want to do. The Physical/ Special move split doesn't exist in Gen 3 and whether a move is Physical or Special depends on the Type. Water Type is Special based so if you want to maximize your use of it, go with Lapras. I find that the most useful ones have the Shell Armor ability.
I usually tend to go with this Moveset for Lapras:
- Surf
- Ice Beam (particularly helpful against Lance)
- Body Slam
- Psychic (particularly helpful against Agatha)
If you decide to go with Gyarados, choose one with a nature that raises Special Attack to maximize your use of Surf (you'll need the extra help, even with STAB and it's base power). Your best Moveset would be:
- Surf (STAB)
- Earthquake (for Coverage against Electric Types)
- Dragon Dance (to set up for a Sweep)
- Return/ Hyper Beam (Neutrality)
Keep in mind that if you go with Gyarados, you NEED to put Earthquake on it, thus, depriving Nidoking of it.
AMAZING REPLY! You are seriously awesome. Taking time out of your day to answer with that much detail. You sir. Are awesome.