If you're a college sports fan, /r/cfb, /r/collegebasketball, and /r/collegebaseball are excellent. The first two are large subreddits, but their mods absolutely stay on top of things. Not just clearing out spam and off-topic discussion, but also posting "official" game threads and post-game summaries so you don't have dozens of "Auburn defeats Alabama 34-28" posts clogging up the front page.