The first Pokémon game I owned was Yellow, so... Pikachu. Not much of a choice, really.
However, I didn't actually get all that far into the game, and somehow got a hold of Pokémon Silver. My starter there was Cyndaquil, and it will forever hold a place in my heart. I was disappointed to learn it has the exact same stat spread as the Charmander line, and I know Typhlosion isn't exactly an unbeatable Pokémon, but a journey in Johto is never complete without my fiery pal!