I mean, that kind of heavily depends on the area of your interests; I don't think that it's really possible to say "forum X is interesting" in a vacuum. I'd add that I still think that there are interesting subreddits on Reddit, though I agree that the front page isn't very appealing these days, at least to me.
On the Threadiverse, though, I would say that as things stand, lemmy is not really good at helping one find existing communities. There's the newcommunities announcement community at !newcommunities, but those, by definition, don't have a userbase when announced, and some of the creators don't do the work of regularly posting content until they catch on. Kbin reccomends random posts in the sidebar, but that's a pretty shotgun way to find things.
What I'd probably do is use the Lemmy Explorer's community search, which as things stand is the only way I'm aware of to search all of the communities across all of the Threadiverse.