Space Quest (and any other early Sierra game) - at the age of 10 or so, these games expanded my English vocabulary many times faster any than other kid of my age learned at school.
Kingdoms of England - the only game me and my dad would agree to play together. I lost nine out of ten times but I still had to try to beat him over and over again.
Wing Commander - during a time when Amiga owners where bullied by their PC friends this game finally showed that the Amiga could pull it off and was still breathing. Well, kind of. Also one of my all time favourite games ever.
Shadow of the Beast - not only was it a game to show to your PC buddies and make them go "Oooh!" but it was also too weird and scary for me to play at that time so I just kept it in the closet and admired the box cover.