I tried to do the opposite once: to rank GitHub repos by effectiveness of contributions, where effectiveness is measured as stars divided by the number of contributions. Sort of, best things done quickly. With all cons and pros of this metric, I've got these projects in top 5:

1. ievms: Automated installation of the Microsoft IE App Compat virtual machines, https://github.com/xdissent/ievms

2. resume: Resumes generated using the GitHub informations, https://github.com/resume/resume.github.com

3. impress.js: It's a presentation framework based on the power of CSS3 transforms and transitions in modern browsers, https://github.com/impress/impress.js

4. deepdream: Neural Network art, https://github.com/google/deepdream

5. favicon-cheat-sheet: cheat sheet to favicon sizes/types, https://github.com/audreyr/favicon-cheat-sheet