It depends on your interests, but for HN readers /r/networking is a pretty good place for enterprise networking discussion. /r/rust has some helpful people in it if you're working with the Rust programming language. /r/science has extremely strict moderation that makes it a good read-only subreddit unless you are an actual scientist who can back up their post.

Generally you want a subreddit with a small but active userbase. Not too small or it tends to die off, but also not too big or the S/N ratio drops off.