And going one further, each topical subreddit usually has a couple of higher quality spinoffs. For example, r/gaming vs. r/games, or r/askreddit vs r/askhistorians.
Couple of other hints for subreddit discovery: if you find one interesting subreddit, check out its sidebar as many have a related subreddits section there. Also check "other discussions" tab for interesting links to see what other subreddits it has been posted on.
Picking your subreddits is a neverending task, I'm couple of years in and still keep finding new and old interesting subreddits.
It completely depends upon your interests, I'm subscribed to a number of music related ones though nowadays I'm wondering why I don't just go back to a message board like AudioKarma...
Not that I know of, but if you like learning and are curious, I'd look into /TrueReddit and any other subs in the sidebar, and /AskHistorians, plus anything in their sidebar.