Keep the scores in a database and use standard SQL to get the info out. Database engines do that all the time and are highly optimized to do it well, fast and correct.

Actually I have a Logitech Slimserver / Squeezeserver (or whatever is its present name). Everytime I play a song and don't fast-forward it to the next one, the server increases that song's "popularity". If I want to listen to the "most popular songs" or even "most popular songs not recently played" (or any other combination of parameters attached to each song), the server goes out to the database and finds them by using some standard SQL.