I have a terrible time trying to remember. I was in 3rd grade with Red came out so I was pretty young. I've probably used each starter at least once which further compounds the issue of remembering which one I used first..
Pokemon Blue - Squirtle ( I remember having my gameboy out in school and having a Blastoise. ) Pokemon Silver - Totodile (I had convinced my dad to buy the Japanese version on ebay for me because I didn't want to wait till the NA version came out. I remember duping Feraligator) Pokemon Sapphire - Treecko Pokemon Pearl - Piplup (I think) Pokemon White - Snivy