If we're including cross-generational evolutions, I'd have to vote the Magnemite line, as Magnezone is one of my top three favorite Pokemon of all time.
For the sake of keeping it purely within Kanto, I voted for Nidoqueen. When I first started playing Pokemon with HeartGold and had no idea what I was doing, I picked up a Moon Stone and immediately tried to use it on the first thing I could. That turned out to be a Nidorina, and she stuck around to make a couple appearances on my team. Then, my very first capture on my very first challenge run, a Yellow Nuzlocke, was a Nidoran-F I named Curium. She turned out to be the undisputed star of my team. As a Nidoqueen, Curium was the only surviving Pokemon on my team when I defeated the Champion and beat the challenge.