Oblivion! Oblivion! Oblivion!!!!
If you're a MMORPG citizen then you should feel right at home in this single player RPG. Battle for Middle Earth II is also strongly recommended.
If you're a fan of the Star Wars franchise, Star Wars: Empire at War is also a keeper.
Of course, a list of must-have single player games is never complete without Civ IV.
And I hear you... I had to wean myself from WoW last year. It was truly an addiction that was an enormous timesink.
It got so bad I threw out the CDs and swore to myself never to play an MMORPG again. They are just too immersive.
I play games to past the time, not to lead an alternate virtual life.