Oh, it's a toughie. I'll cop to Charmander getting overexposed, but I still wouldn't have gone anywhere without my starter, and Charmander was my favourite of the bunch back in the day. I'm less inclined to pick the Fire-type starter than when I was younger, but you know, every child's got a little arsonist in them ready to break out.
Although there's some stiff non-starter competition. I always felt Caterpie looked cute, I loved that weird-looking Abra and was pretty glad to finally catch one of those (curse your Teleport!), and Ralts immediately grabbed my eye back when I randomly found one back when first playing Ruby.
Tell you the truth, I was never good at commiting to 'a fave' even as a kid. There's a long, storied history of me being asked my favourite film and then freezing up.