I have been online since the early '90s, and used to be extremely active on many different forums. I even ran a fairly large forum on my own website, using Simple Machines Forums- which, to my surprise, still exists- from 2005-2008 or so.
I can’t say I have a favourite- I really liked SMF back in the day but I have no idea what it looks like now. I really can say that I really miss the pre-Facebook Internet when there were loads of extremely active forums in every imaginable topic, and people found them and used them instead of just creating a Facebook group and pretending that everyone will just use that by default (I refuse to use Facebook for anything).
I don’t think of Reddit and Lemmy as forums, really, not in the old-school sense. Reddit was a poor substitute, as for many years more weight was put on links than on self-posts, and in the old-school forums I’m thinking of, things like upvotes and “likes” didn’t become a thing until they were used on Reddit and Facebook, and I honestly don’t think having those features makes the Internet better.