The tough part of this is that larger more complex projects usually span enough time that team members and style changes throughout development. This makes the uniformity lower, thus overall quality lower as well.

I attempt to open source everything I work. This forces devs to think about exposure, and this exposure tends to curtail taking shortcuts.

Here are some projects-

Zesty.io’s Admin UI is built with React, styled with SASS and is a reasonable example of a very real world code base.

https://github.com/zesty-io/accounts-ui

Zesty’s manager UI is an example of a much more complex code base that evolved over time. It isn’t what I would call a “clean” codebase but it was always developed with style standards and best practices in mind.

https://github.com/zesty-io/manager-ui

Third And Grove’s Gatsby repo is fairly clean. The standards when I started this project were pretty strict. Styled with emotion, compared to sass or similar approaches I think it’s much cleaner.

https://github.com/thirdandgrove/thirdandgrove-com-gatsby

There may be better examples of smaller projects but honestly, I don’t think keeping a very small project “clean” is much of an achievement. Something you may want to look at are libraries like react-router. Open source heavily used and iterated projects require huge efforts to maintain code quality.

Edit: or, redux :)