While I am asking myself this question, the only one that popups to my mind would be Laravel: https://github.com/laravel/laravel (PHP)

One could think that a codebase as popular as React (https://github.com/facebook/react) would be a perfect example of "clean code" but with a glance, I personally don't find it very expressive.

This may all be very subjective but I would love to see examples of codebases that member of this community have enjoyed working with