You don't have to rank based on single game scores. One way to do this would be to have levels that are replayable (ie there is a number n that uniquely identifies each set of level parameters) and use chess' Elo ranking system for both players and levels.

This gives players a ranking score that is comparable, and over time your players will rank your game levels for you. It also makes it possible to matchmake players to levels.

http://en.wikipedia.org/wiki/Elo_rating_system