All of my favorite ones are banned or removed for one reason or another. For example, one _very_ niche, very entertaining subreddit is gone and it's fanbase scattered to the winds. Died of his death, unfortunately.
I wrote about my favorite software subs recently [0]. tldr; /r/emudev, /r/programminglanguages, /r/reverseengineering, /r/databasedevelopment, /r/esolangs, /r/unikernel, /r/graphicsprogramming.
There were a few more suggestions in the HN thread as well [1] and on Twitter [2].
Two here require some explanation:
Redscarepod is for folks who listen to the Red Scare podcast, though I’d guess 30% of that subreddit listens regularly. It’s my favorite cultural subreddit.
Streamentry is meditation subreddit with focus on attaining enlightenment.