Jeez, you're asking me to remember 15 years back :O If I know myself well enough though, I probably picked Charmander because it looked the coolest. I mean pretty much everyone did that, and I don't think I thought any different :p But yeah, I don't really remember how much I liked it or how far I got with it, but I doubt it was very far, considering I had just about no skill at all, it was most likely the first game I ever played, and I couldn't read English, let alone Danish :p So yeah, I most likely started a new game because I couldn't beat Brock.
The first one I do remember, though, was also with Charmander, though I was just a biit more experienced that time, and I remember getting surprised by how much damage Charmander actually did to Brock's Pokémon, despite being a Fire type. I mean, I didn't think it would damage it at all. I remember getting through quite a bit of the game though, and ending up with a much loved Charizard :D Don't remember if I beat the E4 though.
Nowadays, you'll usually see me pick Bulbasaur. I used to always pick Squirtle because I knew Water was strong against Rock, and it meant getting through Brock was a lot easier. I'd always thought Grass was weak to Rock though, hence why I never chose Bulbasaur. I knew it'd be good against Misty, but I thought I was gonna have a hard time beating Brock with it. Imagine my surprise when I finally use a Bulbasaur and his moves turn out to be super effective against Brock's stupid Onix. Since then, I've used Bulbasaur in most of my first gen playthroughs, since I usually settle for the easy route, but if I want a challenge, I'll usually go for Charizard instead :p I never really use Squirtle, because he's more like the inbetween one. He isn't too hard to get through the game with, but he isn't too easy either. And I just always preferred the two others more haha