I had quite some pleasure working with the Source Engine gameplay code. (Some 10 years ago now)

https://github.com/ValveSoftware/source-sdk-2013

It is not that the quality of the code is high, but just that it is well organized, and everything seems like it was written by a beginner. That makes it wonderfully easy to read and follow the logic.

Since have played around with the Source Engine, I follow the KISS principle with coding with high priority. Rarely trying to be clever, or try to over-do abstractions.