Also, someone mentioned Blue Max for the commodore 64. You can't mention the C64 without mentioning *Jumpman. An amazingly simple, yet fun and addictive game. Also *Summer Games and *Winter Games. Or how about *Temple of Apshai, or *Gateway to Apshai? These were the simplest of rpgs.
But back to IBM based PC's:
I agree with the *X-com sentiment. That was a great game, I didn't like the 2nd as much, and never got to play apocolypse. For some reason I really liked *The Prince of Persia. It was the first game with really fluid movements. It just looked really cool. *Out of this World was really neat too. It had great graphics for the time and the gameplay was interesting. Another great game was *Alone in the Dark. It was pretty much the predecessor to *Resident Evil, another great game. I loved the hand drawn backgrounds and the way the camera angles change while you're playing.
Going back a ways, *Zork 1 2 and 3 were a lot of fun even though they were completely text based. Everyone's mentioned these, but the *King's Quest series was great.
We need more original games now a days. They made them great back in the day.
Also *Star Control 2: The Ur-Quan Masters was one of my all time favorites.

