Mine was Gamefaqs back in 2002. I discover Gamefaqs in the instruction manual for Godzilla: Destroy All Monsters Melee and I join the site when I was 11. I post on the Budokai, Destroy All Melee and Metal Gear Solid 2 section before I got banned when I told them that I was 11. I later re-join Gamefaqs/Gamespot when I was 13 back in 2004.
I used to post on Planet Namek, Temple o' Trunks, Pojo, DB-Unlimited, PlanetAVP and the official gameshark forums (The Yu-Gi-Oh section of the site) around the same time as well back in 2002-2003.