I like both the Civ series and Age of Empires - both are enormous time wasters. Make sure you feed the cat before you start up one of those. The new Civ IV came out, but it has some glitches. I have a good system that exceeds the recommended specs and I am still having problems with it running properly.
I like the Star Wars games, my favorite is still Jedi Academy so far. I have Knights of the Old Republic but I prefer joystick games where you can control the action for action games.
The Harry Potter games are great, Beware of Prisoner from Azkaban. It is visually great but those bozos didn't enable it for a joystick. I don't know about the latest.
The Lord of the Rings series are great too.
I also like flight simulator air combat games. If you like building cities, etc. I would definitely go for the Age of Empires series. I like Age of Mythology and there is a new Titan expansion that is cool.
How about some good old fashioned Chess. Doesn't anyone play chess anymore?