Tie Fighter hands-down. The amount of time I spent on that game during my childhood is insane. Even when my parents got a new Pentium III computer with (at the time) cutting edge performance I found myself returning to the trusty old 486 to fly for the Empire.
I loved flight and space combat games, and I was lucky enough to experience some of the best growing up.
X-Wing,
Red Baron,
Aces Over Europe,
A-10 Tank Killer,
Chuck Yeager's Air Combat, etc. I highly recommend any of these to folks with even a passing interest in the genre (you can get the Star Wars titles and
Red Baron from GOG nowadays). Even wonkier titles like
Knights of the Sky and
Heroes of the 357th had a certain charm, though I wouldn't blame modern gamers if they scoffed at them.
I also recall a submarine game called
Wolfpack. In hindsight it was rather lackluster compared to contemporaries like
Aces of the Deep and
Silent Hunter II, but it was the only sub game I had at the time. It also had a scenario builder, which I toyed around with every now and then. Usually it would boil down to placing as many warships and subs that the game would allow on a collision course and jumping into the fray for some epic sea battles.
And then there was
King's Quest V. That was...yeah...
Well, at least it was amusing, in a twisted and campy sort of way ("A POOOIIISONOUS Snake!")