For me, the top 10 PC games of the decade (by release date):
1. Star Trek: Armada (February-2000)
2. Star Trek Voyager: Elite Force (September-2000)
3. No One Lives Forever (November 2000)
4. Aliens vs. Predator 2 (October-2001)
5. Far Cry (March-2004)
6. The Battle for Middle-Earth (December-2004)
7. Act of War: Direct Action (March-2005)
8. F.E.A.R. (Ocvtober-2005)
9. Dark Messiah (October-2006)
10. Sins of a Solar Empire (February-2008)