There is no absolute favorite game for me. I can give you a list of games that have consumed a great share of my life though, and which I don't regret spending so much time with.
- Starcraft
- Diablo 2
- Unreal Tournament
- Morrowind
- Neverwinter Nights
Those are probably the five games that I've played the most. The list might soon change though, as I will spend more and more time with Oblivion (and its Construction Set) and as I'm finally about to look up the Fallout series (read: I and II). Games that are quite customizable have always interested me the most, so I predict that such games will have a place in my Top 5 for a long time.