I was thinking perhaps this might work. I've listed arguably the most performed works by the major composers in the mainstream to find out which is the most 'popular piece'. Unfortunately this generally means lesser, easier pieces and pieces which everyone would/could play, so things like Scriabin sonata's can't really fit the bill.
This will obviously only end up as being the most popular among piano street users but it could prove interesting.
(if anyone can come up with a 'greatest hit' for Schubert, let me know coz I can't think of one)