For me, it was the AdventureQuest forums. It was a pretty decent first forum, all things considered. There were some unusual but understandable rules (like no one being allowed to upload personal avatars, signatures are allowed but there are heavy restrictions like file size and you couldn't post your signature more than once per page) and the moderation was fairly good (not even swearing was allowed there). It was pretty great for a while, I especially loved the creative community (they got me into doing graphic design for a bit). Though it got way less interesting when people started to realize that Twitter existed and all the creatives started to flock there.
Then the team started releasing all these very samey MMO's (cannibalizing / splitting the player base) while neglecting their other games (like my main game, MechQuest) and I eventually I just lost interest. I think I left right before mobile gaming got really big and basically killed Flash (I can't imagine the AQ team handled that well).
I'm kinda thankful those forums were my first, and that I was also able to find GAF not long afterwards. I'm not sure I could stand using any forums that aren't heavily moderated tbh