Civ, I know that's going to be a little cliche but I just love it.
There's only a select fewgames I can say make me lose track of time completely.
Like most other games make time fly while I'm having fun, but I know I've been gaming for several hours.
The SIms & Civilization are two games where when I play them, I literally pull my head up to look at theclock and realize that it's gone pitch black outside and a full10 hours have passed my by without noticing, not even joking.
The civilization series is just a masterpiece though. Love it!