1. Thief II: The Metal Age
I flat-out adored this game. Must have replayed it at least 20 times. The stealth mechanics, feeling like a ninja, this game just worked to perfection. As a consequence, this game also led me to the Splinter Cell series, but I didn't recapture the magic.
2. Forza Motorsport 3
This game, coupled with Forza Motorsport 2, are my most played games to date. They may not have an engaging career mode or great sense of speed, but boy howdy is the racing fun. I love cars, and so does this game. Weirdly enough, its also the only game that can send me to sleep while playing it.
3. Sid Meier's Civilization III
Biggest time suck in the history of gaming. I have also played the 2nd, 4th and console versions of Civilization, but this is the golden game in the franchise for me. An unexpected side effect was its sparking of my interest in ancient history.
4. Mass Effect
It seemed like such a sleeper hit to me at the time, as I never heard that much about it prior to its release. Now it's part of one of the biggest franchises in gaming. I played this game through many times, and it was the first game that had me literally jumping out of my seat in excitement over the plot developments. The sequel was initially disappointing by comparison, but I eventually came to like it.
5. Need for Speed III: Hot Pursuit
Another racing game, one of the best Need for Speed games there ever was.
6. Worms Armageddon
A hilarious silly game, and the best in the worms franchise. I was one of those players that was crazy good with the ninja rope. Recent Xbox Live Worms games haven't been able to scratch the nostalgic itch.
7. Mario Kart 64
I never owned a Nintendo 64, but I still managed to sink a lot of time into this game at relative's places, and later at university.
8. Monster Truck Madness
MTM for Win 95 was another racing game I loved. The sequel was disappointing, as it featured WCW themed trucks. Why was wrestling associated with Monster Trucks? Seems obvious now.
9. Quake II
Probably the first FPS that I really enjoyed. There have been better FPS' since, but this game was the one that started it all.