My first starter was Charmander in Blue, but I used Squirtle in FireRed, and I've used all three before in different games hacks or official. I have to say despite 4x weakness to Rock, gotta love Charmander. It's one of my favorite colors and has fire on it's tail, Fire types are my second favorite, and it's just so cute. Pikachu in Yellow is ok but I hate not bejng able to evolve it and still have them follow you.
Squirtle is a close second, as he got me through many hard times in FR, but I'm still not that big a fan of Venusaur. I even used one in Y, but..... Well they aren't too cute(I prefer Ivysaur) and I don't really like all of it's moves(but Leech Seed.... Yes....) but I'm sure I might like then better in Red/Blue, seeing as I've onlh used him in Y and Yellow, once in RB but I lost that game before going to Viridian Forest.