For me, it was RPG Town and GamingGroundZero. I had perused RPG RPG Revolution, rpg2knet, SkyTowerGames, etc. but I frequented (and acted like a stupid kid constantly on) RPG Town. It still exists but it's been abandoned and the bots have overtaken it.
While not a forum, DevScape was a cool place for uploading and showcasing your games. Also I found the original RPG Palace from Angelfire yesterday haha.