1. Elite 2 - Had it many years ago on the amiga then got it again years later when I had PC, no other single player game has had the amount of hours that I dedicated to this.
2. System Shock 2 - I know it's showing it's age these days, but back in the day I honestly had to stop playing this in the dark, the whole game oozed atmoshpere.
3 The Half-Life series - Not much to say about this other then I've enjoyed everyone from beginning to end.
For an honourable mention I would have to say Anarchy Online, pales in comparision to the likes of WoW these days, but it was my 1st MMORPG and I've always been more of a sci-fi fan then fantasy. I met some great people back in that game, most of which that I still keep in touch with.