Updated list of my favorite rooms in no particular order.
Trivia | The first actual room I joined on Showdown. I used to think I knew everything about this game. But boy, was I wrong. In this room I made a lot of friends which is always nice. It was a very special room to me and I loved answering trivia questions. Expanding your knowledge about a game you've played for basically your entire life is a very fun thing to do. Especially when you realize there's so many things you didn't even know about it.
Cosmopolitan / Fashion | Fashion is something I always was into and Cosmopolitan, Fashion at the time, was the room that allowed to me to share fashion and ask for opinions. I loved doing it. Later on the room merged with the Wildlife room, which I also liked a lot. Animals is one of my other hobbies. And that's why I loved the room.
Game Corner | Another room I loved. I loved playing chat based games, rather than chatting all the time. Seeing as how I wasn't keen on competitive Pokémon back then. I was Making a few games and seeing people enjoy them and it going well was a great feeling. Made a lot of friends, some of which I still have contact with these days.
Monotype | The Monotype format was something I always had found interesting but I always thought I would dislike it because I didn't feel comfortable playing competitive Pokémon. However, I was pleasantly surprised. I joined the room in hopes of learning more about the format. What I got out of it was something I didn't expect at all. I made a lot of friends in the room and felt like I was part of them. They were the most friendly and caring people I ever met. I was allowed to play in the room league and was the leader of the Normal type.
Art | When I really got into spriting I had to join this room and see if more people were spriting like I was. Luckily there were quite a few. And even though the room didn't centralize around spriting, the discussion was enough to satisfy me and make me stay. Met more cool people some of which I am still on good terms with. I occasionally still join the room asking for help. Because, who can help you better than experienced spriters?
Nederlands | A room for people that spoke the same language as me... That's what I needed because I didn't quite master the English language. And because there was no set topic it was also very free. It allowed you to talk about almost anything you wanted. Just like most language rooms. Discussing topics which you liked, that the foreign people didn't understand, was great.
Deutsche | I don't quite remember how I got into the room. But I love this room. Despite me not speaking German. Luckily for me they also allowed English to be spoken in the chat. A whole lot of cool people to interact and talk with. I started liking the room so much that I volunteered to write the article for the room. Which I'd think turned out pretty well. I still pop in occasionally but am not as active in it as I once was.
Help | Helping people is something I always loved doing. So this room was a no-brainer. The appreciation after helping someone is something I loved experiencing. And therefor I decided to stay with the room.
Cafe Le Wow | This is my home. I basically live in here. I met some cool people in here and I stuck with it since I joined a few years ago. Everyone likes foods and drinks but not everyone likes getting hungry by the pictures and links the room staff posts.
Tournaments | Even though I said several times that I am not much of a competitive Pokémon player, I still love this room. Even people like me, who do not tend to ladder very much, have urges to participate in tournaments just for fun. And that's exactly what this room is, fun. Many cool people in here that are willing to battle you, chat to you and laugh with you.
The Studio | I am kinda surprised I never mained this room before. Because I love music and I think there's a ton of people out there who do as well. Yet I joined relatively late. A lot of cool people again in here and it's awesome to expand your comfort zone when it comes to music. I now listen to a lot of songs, bands and artists I never expected to like. Woo, way to go Studio!
Survivor | I'm a survivor, I'm not gon give up, I'm not gon stop, I'm gon work harder. I'm sorry. I could not resist it. Anyways, I like the Survivor room because it's got fun people and a lot of funny games as well. It's basically Pokémon but in text form. Almost everything is decided by a coin toss which I like.
RarelyUsed | Again, I am not much of a competitive player but Sun and Moon RarelyUsed is one of the funnest tiers I played. A lot of fun to use Pokémon in a very balanced metagame. And then you realize it's even still in beta(?). The room is very laid back and chill and has a lot of funny and friendly people in it. But also a ton of terrible puns. And that is why I love it.