Based on the criteria of oldest computer game you actually like, Beneath Apple Manor 1978 Apple II. It's on a PC, I like it well enough, and it's the oldest fitting the criteria, failing that Smooth Max 1979 Apple II which I like better but Beneath Apple Manor seems to fit. I liked Zork which came out in 1977 but it came out on a PDP-10 mainframe.
Probably Colassal Cave which I played as Microsoft Adventure. It was originally released for S100 systems sometime around 1975 if I'm not mistaken, but the PC version, Microsoft Adventure, was released in 1981 and was a booter. Still have a copy I run on my Tandy 1000.
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Maniac Mansion or Last Ninja... Both from 1987.
