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.
The entire Commander Keen franchise.
There's just something about the pre-Doom era of PC gaming that still appeals to me. Maybe it's the "golden age of sci-fi" aesthetic, which seemed to promptly vanish from PC games after Doom was released (but quietly came back in the PS2 era, thanks to Ratchet & Clank.)
Anyway, apart from the 'obvious' ways in which it hasn't aged well (graphics, sound, gameplay style, etc) some of the flavour text in one of the games contains an (almost jarringly nonpartisan) George H. W. Bush joke.
It was, and still remains, a bit puzzling to me that Commander Keen episodes 4-6 and Doom had basically the same dev team (Carmack/Carmack/Romero/Hall/Prince). I have no idea what was going on behind the scenes during that period but from where I'm standing it represents a "David Bowie in the 70s" level of self-transformation.
this song is pure nostalgia
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.