Nidoking prefers using a Timid Nature with Speed Evs even if its a Stealth Rock Lead. This is due to the fact that Nidoking will want to outspeed as much as it can. Lets look at Nidoking's Max Speed. With 31 Ivs and a Speed Boosting Nature it has 295 Speed. Without any Investment Nidoking will have 206 Speed, now I will list some calculations.
252+ SpA Choice Specs Primarina Sparkling Aria vs. 252 HP / 4 SpD Nidoking: 600-708 (163.9 - 193.4%) -- guaranteed OHKO
252+ SpA Choice Specs Volcanion Steam Eruption vs. 252 HP / 4 SpD Nidoking: 746-882 (203.8 - 240.9%) -- guaranteed OHKO
These are calculations from Pokemon that Nidoking can usually beat since it runs max speed and kill them with Sludge Wave and Earth Power respectively. Its not uncommon for these two Pokemon to run Modest with 252 Speed so these can faced in competitive play. There are some other Pokemon that Nidoking will appreciate being able to outspeed and KO but these are two that first come in mind.