Personally, I'm in the Sonic Fan Games HQ and Sonic Stadium.
I've joined SFGHQ in 2022 but only did my first post a couple of weeks ago though somehow it's even emptier than this one. As for Sonic Stadium I joined like a week ago because I wanted to be in somewhat active forum. It's pretty cool but this forum kinda feels cooler to me. I don't really know why but there's something about a forum for a sub-series like Wario that I found really neat ! Maybe, it's because if the fact that it's a less talk about franchise, I don't know. That's why I wish this site was more active.
I sometimes go on GBATemp and GameFAQs, especially when I'm researching some video game stuff but never actually singed up. I don't feel like I'd enjoy them much, maybe I'll check them out one day.
Also forgot to say but I'm also of Mario Fan Games Galaxy (I think I'm named Dzario in it) though didn't post anything yet. There is an account there called NacklesK and it's mine but I had trouble with it so I made another one.
When I was younger, I used to be on Mario Fans Game Galaxy, Super Mario Bros. X., Minecraft, and some Animal Crossing one that I don't remember the name of.
I used to like Technodrome Forums. It was a great place for TMNT fans to discuss comics, movies, video games and cartoons. And even a couple writers who worked on the comics were active on there.
But time did a number on that place and most of the people still active on that forum are just toxic as hell.