For a C codebase, Postgres[1] wins for me hands down. It's clean and suuuuuuper well commente, such that with a little context you can dive into something very complex and still get a feel for what is going on. [1]: https://github.com/postgres/postgres
Second this; Postgres codebase is what got me out of the "good code is self documenting" nonsense. For those of us in the database space it is an incredible resource - and overall a great example of good code. sqlite is much less complex, but similarly approachable. In more recent examples, I think you see a lot of this same reader-centric pragmatic ethos in many Go projects. The Kubernetes codebase comes to mind as…
Man I hate dogma like that. My "common sense" comment style is always, "code tells me how, comments tell me why." The only exception is in hand optimized code where I'll non-doc comment what the reference implementation would be above the optimized version, which is _sometimes_ necessary when tests aren't in the same translation unit.