Formerly IRC, now Matrix, discord, slack etc, with a small community that actually likes each other is pretty good, but that's a shitty answer to the question, because invitation into these circles is almost always friend-of-a-friend.

The various rationalist spaces are at worst filled with pedantry, and at their best home to useful in depth discussions on very niche topics.

Some forums are good, but the majority seem to have a serious problem with the user base aging up and becoming very cliquish. I actually wish I knew why this clique behavior was so much more common on message boards than just about any other comms medium.

Reddit is mostly pretty bad, and as you mentioned seems to have poor moderation down to a near science. I actually think a lot of particularly bad mod practices and also common hostile mod-user interactions are driven by Reddit's global karma as a primary or secondary effect.