One problem is that a few communities, while they do show up in my subscriptions list, do not indicate that I am subscribed to them when I am on the community page (for example, !asklemmy@lemmy.ml is one of those communities where I noticed that issue, it mostly fluctuates between me being subscribed to the community or not). If it needs to be marked as an issue on GitHub, you can let me know.
Lemmy.world, because it is the instance I am most active on and read the most posts on when it comes to the many different instances I’ve joined. I also find the feel of the community there quite welcoming.