There should be a list of the top 100 listeners for each genre/artist etc.
Possibly a ranking of most listened-to playlists or largest playlists
Users with the most time on spotify in terms of minutes played per week/month/year
Update: This idea has been reposted here.
There are multiple statistics suggestions here. Please submit them separately after searching to check that someone hasn't already suggested the same thing.
The first one may be impossible to implement for user privacy reasons.