You can't effectively rank developers. First there are just too many to rank. Even in college football, where thousands of people are paid everyday to monitor it, they don't try to rank all of the ~119 Div 1 teams, just the top 25. Secondly there isn't a simple metric to rank developers. It's about as smart as saying look I did the most work on this project because I wrote the most lines of code.

This could even have a negative effect if developers get concerned about their ranking and try to game the system instead of making quality contributions to projects.