I second the Animal Crossing community! I started playing New Horizons as a way to relieve the large amounts of stress I've been under, and everything I've seen from the community so far has been just as wholesome as the content of the game. @Pifanjr
same goes for Stardew Valley as well. Wonderful people.
There are lots of communities I could mention, but I'll stick with two for this post. I'll start with the game everyone is probably tired of hearing me mention:
The Kerbal Space Program community. Even players who have poured hundreds of hours into the game still remember the challenge of landing on the Mun for the first time, and the sense of accomplishment they felt afterwards. Players who have been to every corner of the Kerbol system still welcome newcomers who are trying to make it into orbit for the first time, and offer sincere congratulations when they succeed. The game is all about stretching yourself to achieve new goals, even small ones, and the community is very supportive of that.
Even though this one isn't a specific game, I'd like to mention the speedrunning community as a whole. You could pick any of the most popular games and this would apply. I've had the most experience with the Super Mario Odyssey speedrunning community. Everyone is working towards the same goal of beating the game as quickly as possible, so even though players are competing for the best times, they also want to see the record get lower and lower. It's not uncommon in the more popular speedrunning games for a player to put a cash bounty out on their own record, hoping someone will beat it by discovering even faster strategies. People also put tons of time into coming up with beginner routes and tutorials for new players to help bring more people into the community. As I've found with many niche communities like this, most people are just in it for the love of the thing and want to see their community thrive.