Strictly talking about code quality, I will nominate RCP100, which is a small, virtually unknown, now-abandoned routing software written in C [0]. I started programming with C way back in the 90s, and this is one of only two projects I can recall being immediately struck by the beauty of the code (Redis being the other). I know almost nothing about the author but he seems not to want to be known by name. You can browse the source on Github [1], which I uploaded myself, since you can only get a tarball from sourceforge. Anyway, as someone else mentions, C is usually a mess, but RCP100 struck me as beautiful.