Officially, my first pokemon was a Pikachu in yellow, though I don't actually count it as my first, since back then, I was just a toddler. I couldn't read very well, so I didn't know how to save the game. Every time I wanted to play, I'd have to start over. I never got much farther than Viridian Forest, usually. While I did go back and actually play through and beat yellow when I got a bit older, I always consider it as the second pokemon game I played.
When I was around 6 or 7, I got crystal, and I obviously could read quite a lot better by then and didn't struggle with the basic game mechanics anymore. My starter was Chikorita, who i named Janey after my best friend at the time. However, back then I didn't know which gender sign was which, so I mistakenly thought Janey was a girl the entire time. (And allllll my female pokemon had boy names, and alllllll the males had girl names.)
Anyway, I as a dumb idiot child, didn't really understand type advantages, the purpose of defensive moves, or why I would ever bother training a pokemon other than my starter. So by the time I got to the elite 4, I had Janey, my now extremely over-powered Meganium, a Ditto, a Pidgey, a Sentret, and I think a couple other first route, unevolved pokemon in my party. I beat the entire e4 with just Janey. I beat the entire game with just Janey. It was extremely difficult for me, and I remember it taking me a couple of years to complete everything, but now as an adult I look back at it like, dang, I'd never want to attempt a pokemon game with just one pokemon in recent years.
After I beat the game, I stupidly let my cousin borrow it, and she saved over my file, so Janey is gone now. I have a new competitive Meganium in Moon named BUTTHOLE who I love dearly and am constantly considering renaming to Janey in my lovely baby's honor. RIP Janey u are missed.