The first one overall? I can't recall.
First one significant to my memory was BumbleKing. I came there at the absolute worst time, unfortunately, but I've met some people that led me to other places I go to to this very day.
That forum is locked; no new members can join, and existing members can't post either. Given the state the community had gone in, I'd say it was a mercy kill, but we'll never know.
Still, can't regret it with the people I've met, and while I was rather immature when I started, I like to think I've matured a bit since then. Looking at some of my older posts does make me cringe, though.