Postgres. I don't code in C if I can avoid it, since it often feels like an awful lot of extra typing while still having to worry about memory safety. But the Postgres codebase is extraordinarily well organized and respects the humans that work with it with its intelligent handling of memory and judicious use of macros.

I consider the core Postgres codebase to be the gold standard in development even though it's in a language I do not prefer to write in if given the choice.

Shout out to the pgrx folks. You're awesome! https://github.com/pgcentralfoundation/pgrx