Half-Life 2: Continued the story of Gordon Freeman and also delved into Black Mesa's history prior to the incident. Physics-based combat and a story that is (in some ways) better than most sci-fi films*.
Grand Theft Auto: San Andreas - Vice City ate up my time, but San Andreas ate it, puked it up, and then ate it twice more. No game has taken more of my hours than that one.
Bioshock 2 - Bioshock was already a head spinner, but Bioshock 2 really turned you on your ear. Looking forward to Infinite.
Age of Empires 2 + Conquerors expansion - AoE was my first introduction to RTS games, and AoE 2 took a major placeholder in my RTS life. While AoE 3 had physics and better graphics, the units and environments were not as diverse as AoE 2.
*Totally my opinion. Don't flame/rage/whatever. Plz