UT2k4. The game is still a masterful blend of balance and fun, and I spent a good 4 years of my life playing it. Hands down one of the best, if not the best, arena shooter to be made.
CoD4 has a special place in my heart for where I met most of my online buddies, and we've been through almost 5 years of gaming together since. Also the shooter I credit for letting me realize I'm not ass at fps. Those buddies almost banned me when we first met cause they thought I was hacking. Good times ^.^
And I can't decide, but Pokemon, Metro, Civilization/Alpha Centauri, and League of Legends all sort of glob up in some weird 3rd place monstrosity of indecision. Either childhood, feels, or pure hours logged makes all of them so hard to pick favorites over the other.
Followed by Harvest Moon: A Wonderful Life and LoZ: Wind Waker in another bout of indecision. Because the Gamecube was the best fucking console ever made, you shut your whore mouth.