Nidoking shouldn't be compared to Landorus since they check different things. Notable advantages Nidoking has over Landorus include:
-Strong secondary STAB in Sludge Wave, while Landorus doesn't have a secondary STAB to use;
-Fairy resistance, meaning it can safely switch in against the likes of Clefable and Sylveon;
-no 4x Ice weakness, so it's not destroyed by random HP Ices and can survive Ice Shards;
-Toxic immunity and SR resistance, so it fares better against stall (both are forced out by Cresselia and Chansey anyways, but Nidoking can switch in and out more easily);
-Better SE coverage granted by Fire Blast, Ice Beam and Thunderbolt. If your team needs to handle specific threats with a 4x weakness such as Ferrothorn, Gyarados and Dragonite, Nidoking is the superior choice;
-Doesn't rely on Focus Miss for coverage.
These advantages are undeniable and make Nidoking stand out, so under any circumstance should it be just dismissed as an inferior Landorus.
-Strong secondary STAB in Sludge Wave, while Landorus doesn't have a secondary STAB to use;
-Fairy resistance, meaning it can safely switch in against the likes of Clefable and Sylveon;
-no 4x Ice weakness, so it's not destroyed by random HP Ices and can survive Ice Shards;
-Toxic immunity and SR resistance, so it fares better against stall (both are forced out by Cresselia and Chansey anyways, but Nidoking can switch in and out more easily);
-Better SE coverage granted by Fire Blast, Ice Beam and Thunderbolt. If your team needs to handle specific threats with a 4x weakness such as Ferrothorn, Gyarados and Dragonite, Nidoking is the superior choice;
-Doesn't rely on Focus Miss for coverage.
These advantages are undeniable and make Nidoking stand out, so under any circumstance should it be just dismissed as an inferior Landorus.