Couple of my favorites:
* https://reddit.com/r/AskHistorians - All things history. Quality of discussions are way better than on most other subreddits. Highly moderated
* https://discuss.bootstrapped.fm - bootstrapped businesses
* https://lobste.rs - kinda like HN but for tech only
* https://slatestarcodex.com - it is more blog than a community, but great nevertheless
I used to frequent other communties but these are the ones that stood the test of time
Besides HN, a handful of subreddits (r/MachineLearning in particular), and I used to spend a decent amount of time on Product Hunt.
I do miss the hell out of forums though. I feel like i "knew" everyone on a better level and could burn through hours talking.