If I had to pick one, it would be Doom. Honorary mention to Quake, Starcraft, and Myst. Everybody I know has played Doom and Myst, Quake and Starcraft are just that good. --- Against my better judgment, I have decided to put my MAL URL here: http://myanimelist.net/profile/rtkamb
Newell: I was at Microsoft for 13 years and, to use games industry parlance, I was the producer on the first three releases of Windows.
One of the problems that Windows was perceived as having, between going to Windows 95, was that it wasn't a good gaming platform.
So around the time that Doom shareware came out, I installed it on a laptop and dragged it around everybody's office and said, 'Look, look what PC games can do! This is a lot better that your NES system or your Sega system', and decided to have some engineers work on porting Doom to Windows.
I called John Carmack and said, "Hey, we'll do this for free". And eventually it became the Doom port to Windows.
During the course of Quake development, a friend of mine at Microsoft moved to id to work with John [Carmack] on Quake - he was one of Carmack's programming heroes. So he'd gone from Microsoft to id and the two of them said to myself and Mike Harrington, another Microsoft employee, 'Hey, you guys should stop working at Microsoft and start a games company'.
We went down there, must have been the summer of 1996, and bounced around some ideas with John and he said 'Great, here's the source code to Quake, go build a game'.
Mike and I looked and each other and said, 'Well I guess we're going to start a games company now'. That's how we got started.
So right there, doom indirectly saved PC gaming by causing Valve (and thus Steam) to come into existence.