/r/askhistorians is regarded as pretty much the "perfectly moderated" subreddit others strive for

/r/science and /r/programming aren't bad

After that you should just check out niche communities and see if they've been taken over by trolls. So far /r/motorcycles, /r/ultralight, /r/camping etc have been fine, as opposed to the many iterations of /r/onebag which generally suffer from power-grabbing moderation.