Let's see...
I remember that I've been participating on the forums (such as Creatures Caves) for a relatively obscure franchise called Creatures, made by Creature Labs. At that moment, I was 8 years old! Which is why I don't remember much aside of myself dreaming on integrating Yoshi, Koopa and Bowser as the new species. That and getting temporarily banned for illegally bringing download links for more Norn races while being completely unaware of the rules. These communities still exist, but all of my posts were gone after they transferred to another server, so I saved myself from further embarrassment. But still, that was fun.
Then, there's NSider from the official site of Nintendo of America. Nothing much to say here, but the staff not allowing us to share friend codes is a real bummer, because the Japanese giant was still waaaaaaaaaaaayy behind when it comes to understand on how Internet works. I'm still embarrassed with the posts I made there as JC5555, but nowhere near bad as when I was much younger.
This is why I went to GameTalk Forum. I could post here without having to go through the Join page, and this is when more of my first successful Animal Crossing: Wild World Wi-Fi sessions were done, albeit with the snail-paced lag-fest dial-up connection. I would frequently end up disconnecting too many times, but I was apparently hyper patient to deal with it. Anyway, the forum's UI is kinda cheesy by today's standards, but it's still practical to me. The official forum is forever down since 2008, but there's an alternate GameTalk forum with the nearly identical UI is made by someone else.
Next, an old friend of mine suggested me to join AXA Forums. It was one of the big 3 Animal Crossing communities by the time I joined and Animal Crossing: City Folk was announced. The staff there was a bit stringent with the rules, so the learning curve is kinda high. There was even no linking to YouTube videos! That didn't prevented me to stick for a longer while, mainly thanks to its charming members. Unfortunately, the forum started to go downhill when the first server crash happened in late 2007 and had to create a new forum from scratch. It lasted a few months, but that was enough for it to cause a huge amount of members to never come back again. It repeated again in October 14th 2009, because someone figured out what was the password for one of our administrators and made a mess. The forum was disabled and it shouldn't take long to clean it up. Problem is, the master admin forgot to reactivate the forum and it took 9 months for it to become fully operational. Meanwhile, a massive amount of users were gone and I ended up joining TBT 1.0. As time gradually passes, AXA Forums was falling further and further, and not even Animal Crossing: New Leaf could save the day. And all of that was due of the lack of presence of that master admin, therefore not encouraging us and the staff member to fully involve on a dying forum. It eventually became a ghost town around the time Fall 2014 arrived. Oh well, this is quite possibly the most memorable experience I had with a forum.