The one game that I got the most out of just happens to be my favorite one of the bunch. I first started played TF2 back when the beta came out and I've been addicted ever since.
Before TF2 stole my heart and soul Counter-Strike 1.6 was my game to play whenever I had some extra free time on my hands. The sense of community some of the servers provided kept me coming back for more.
My favorite First Person Shooter series, the size of the maps and the style of gameplay which was revolutionary at the time immediately drew me in. I can drive a tank off a cliff into the water, swim out to the enemy's aircraft carrier and steal one of their fighters? Sign. Me. Up.