My list:
2000: Mario Party 3
2001: GTA 3
2002: GTA Vice City
2003: SOCOM 2 US Navy Seals
2004: Star Wars Knights of the Old Republic 2
2005: Gun
2006: Elder Scrolls 4-Oblivion
2007: Mass Effect * Honorable mention Bioshock
2008: Fallout 3 * Honorable mention Civilization Revolution
2009: Batman- Arkham Asyslum * not definite until 2010
GAME OF THE DECADE: Oblivion * Honorable mentions to Mass Effect and Knights of the Old Republic 2.
This list is based on multiple factors, including Fun while playing, The platform I had access to, I actually played the game, and Amount of time I played.
Games that I did not play, or did not have access to, did not make the list.
Also, it is important to note, that my video game experience was extremely limited until 2005, and I am not including multiplayer as part of the criteria.