Being an Elder Geek I have a bit longer span.
The Tower of Myraglen may never be equaled for innovation. Unlike any other game, you had to change your computer to complete it - or do some very strange things to your life. You had to reset your time, and date, etc and if you really wanted to play, change the CPU clock..
Since it was on the Apple][ GS you had complete control of the hardware and could even edit the RAM to change the game. While it was running! I truly miss the old days of being the master of the computer.
Second place is Diablo2
Third is Starcraft
Lately I play a ton of WoW, but it is not my favorite.
Like Windows, it is what I use and tolerable. But not my favorite.