The {BSD/OS, Free and Net}BSD sources in the mid-90s, coming from Linux and a source license for a commercial unix, both of which were mostly undocumented and didn't have any version control. The commercial unix was really grody, with lots of copy/paste of the same function with different bugs each time, and comments with bug numbers that meant nothing to us and didn't explain a thing.

The BSD sources were sanely laid out, documented and had a beautiful coding style that made the code easy to read. Free- and NetBSD even had version control.