http://www.reddit.com//r/AskHistorians/ is easily my favorite subreddit. The quality of the lengthy submissions and the fanatically rigorous moderation is amazing.

http://www.reddit.com//r/DepthHub/ is a great highlighter for introductions to quality subreddits. Its how I found /r/AskHistorians/

http://www.reddit.com//r/BestOf/ is good too.

Other than that it depends on your particular interests.

There is an outstanding problem that subreddits I would love exist but are hard to find. However that does also protect them and their mods from the slavering hordes of captioned gif posts.