Ask HN: What open source project, in your opinion, has the highest code quality?

The only core library code I needed to look at was namedtuple, which is pretty incomprehensible even with context.

"Highest" I don't know, but "code whose quality I look up to", then:

For C: Process Hacker and some similar code that is designed like and written around Windows kernel APIs: https://github.com/processhacker/processhacker/blob/master/p...

For C++: Some of the Boost code, and stuff like it, such as P-Stade Oven: https://github.com/himura/p-stade/blob/master/pstade/pstade/...

For others: (need to look later, I forget)

Definitely agree with this. Both the documentation and code are of excellent quality. Others that come to mind are sqlite and zeromq.

I hold the source code of Go standard library & base distribution (i.e. compiler, etc.) in very high regard. Especially the standard library is, in my opinion, stunningly easy to read, explore and understand, while at the same time being well thought through, easy to use (great and astonishingly well documented APIs), of very good performance, and with huge amounts of (also well readable!) tests. The compiler (including the runtime library) is noticeably harder to read and understand (especially because of sparse comments and somewhat idiosyncratic naming conventions; that's partly explained by it being constantly in flux). But still doable for a human being, and I guess probably significantly easier than in most modern compilers. (Though I'd love to be proven wrong on this account!)

At the same time, the apparent simplicity should not be mistaken for lack of effort; on the contrary, I feel every line oozes with purpose, practicality, and to-the-point-ness, like a well sharpened knife, or a great piece of art where it's not about that you cannot add more, but that you cannot remove more.

SQLite.

and for this reason alone!

https://www.sqlite.org/testing.html

    As of version 3.23.0 (2018-04-02), the SQLite library consists of approximately 
    128.9 KSLOC of C code. (KSLOC means thousands of "Source Lines Of Code" or, in 
    other words, lines of code excluding blank lines and comments.) 

    By comparison, the project has 711 times as much test code and test scripts - 
    91772.0 KSLOC.