I think I got to go with Steelix. I always thought Brock's Onix was just so cool-looking, and definitely remember the tough love he gave me in Pokemon Yellow-- practically flaying me alive.
Then in GS Onix got this intimidating-looking evolution that made him a lot more solid as a Pokemon with the steel dual typing. He really appreciated the stat increases, especially that beautiful defense and so many nifty new tools from curse to roar. Over the years he continues to steadily find ways to better himself from his new abilities in gen 3 like sturdy to his mega in gen 6. Did I mention how much I love those crystals on his back?
Jasmine is my favorite Johto gym leader by far. She is beautiful on the outside and within. She teaches you about empathy and perseverence, and having a Steelix as her signature Pokemon is just awesome. All that steel hidden under the velvet, beauty and the beast.