For me, I'd have to say Old School X-COM (UFO Defense, TFTD, X-COM Apocalypse to an extent). I love them in a way that I just do not enjoy NewCOM, but the interface was quite possibly based on the recovered works of a 13th century scribe, and is very difficult to get used to for an unfamiliar player.
for the others, perhaps. for carmack, he never gave a single damn about the actual games. he liked tweaking engines.
with keen, his interest was the smooth scrolling he was able to squeeze out of shit hardware. the i bet the entire rest of the game occupied 1% of his attention or less.