Halo: Combat Evolved blew me away when it first came out. I was introduced to a whole new sci-fi game that had an interesting plot that I could follow. It was also revolutionary at the time, perfecting concepts such as regenerating health, holding 2 weapons at a time, adaptive and smart enemy A.I, vibrant and masterful music that enhanced the gameplay, unforgettable enemies, and a bunch more. The multiplayer was just OKAY on the PC, but the single player amazed me, even though I spent probably 5 hours of my whole life back tracking my way across the bland level environments.
Halo 3 was also very cool too. It had its funny and awesome moments, such as killing someone who JUST instantly stole a flag from your elephant with a spartan laser from across the map. The graphics and the environments were a huge improvement from the other games. Also the multiplayer is more fun on Halo 3, and the return of a more powerful assault rifle made it even better.
Call of Duty 4 is simply a masterpiece. I was always looking forward to a modern warfare game that would perfect the sub-genre to its limits. I didn't know that Infinity Ward could pull off such a feat, after I had lots of fun with Call of Duty 2's WW2 single player game.
Now I can only make a list:
Gears of War
Starcraft
Warcraft 3
Counter-Strike: Source
The Orange Box