This topic makes me notice that while I come across many Discourse instances in the wild, the only ones I actually post in are meta.discourse.org and my own forums.
So I don’t really have a “favourite one” in terms of features, design or even community/activity.
I’d say that my favourite Discourse sites are the ones I migrated (whether I’m active on or not) that are all ancient forums, some more than 20 years old.
My main forum (unicyclist.com) even has content as old as 1993 (imported from newsgroups hosted by the original admin) that is still available. Discourse is one of the essential links in the software migration chain that allows communities continue to exist online even after more than 30 years.