Q3A is probably one of my favorite codebases to have had the honor to work with. There's just so many places where you get these holy shit, that's a brilliant idea moments. I don't know what Doom 3 looks like under the hood, but Quake 3 was a real marvel of engineering as much as anything else.
Quoted from Zachary J. Slater, Lugaru for Linux Ships; Gordon Updates Plan LinuxGames (2005-08-26)