A Gen I original, Nidoking kicks off the top 10 list. During my first Red runthrough (and most subsequent plays) I attempt to catch a Male Nidoran pretty early on in the game just so I can use this bad boy. This guy's so cool that even the boss of Team Rocket/the final Gym Leader uses one in his own party.
The introduction of Hidden Abilities in Generation V really allowed Nidoking to shine in competitive play. His ability, Sheer Force, grants him a bonus to all of his attacks in exchange for nullifying their secondary effect (for example, Ice Beam will never freeze anything), but it's a small price to pay for such devastating power.
During my playthrough of X, I captured a shiny Nidorino (middle stage evolution) and immediately evolved it into Nidoking. That was my very first ever legitimate shiny, and he's still part of my collection (His shiny form is one of my favorites).
A staple of my Generation I crew, Nidoking will always hold a special place for me.