Hmmm...
Pokemon Red, back when I was eight years old. I recieved it for my birthday and up until that point I had no idea that there were Pokemon games. I started with Charmander, got frustrated that I couldn't beat Brock, restarted with Squirtle and that kicked off my love for the water starter. Ever since then, I've always picked the water started in every Pokemon game I've played (Totodile for Gold/Silver, and Mudkip for Sapphire).
Speaking of Mudkip... I've had the same game since 2003, and I honestly believe my Level 98 Swampert and I have bond. I'll more than likely actually cry when the time comes to get rid of Sapphire...