Quake 1 is most likely the simplest and most straightforward though it does have a bunch of alternative things for stuff like GLQuake vs software rendering. Qu

Quake 1 is most likely the simplest and most straightforward though it does have a bunch of alternative things for stuff like GLQuake vs software rendering. Quake 3 was designed to be used by 3rd parties and is more well put together, focuses on OpenGL and Carmack considers it his best C code. I'd say one of these two.