If I had to rank them, I guess it'd go like;
1. Bioshock
2. Halo CE
3. Halo 2
4. COD World at War
5. COD Modern Warfare
Possibly unpopular opinion; as great as the 2000s were, I actually think the 2010s might end up being better in terms of FPS games by the time they end at the close of this year, once Doom Eternal is out.