Ask HN: Best Codebases You've Worked With?

The Linux Kernel, by far. Once you understand The Way the Kernel developers writes code, you can understand a lot of code you've never read before because they use the same patterns. I've worked with a lot of open source stuff and none is as neat as the Kernel.

An internal framework for mobile apps that the CTO of a previous company had built. Very simple and very flexible surface that hid a lot of complexity of persistence, offline state syncing and conflict resolution.

Spring

I second this, but I'd say really only the core framework.

If you look at some of the peripheral spring projects, the quality is not quite as stellar.