I feel like Call of Duty is for the kids who aren't good enough to compete in Halo or CS. I hate when they're like oh CoD is so good!!111! just because they can't play any other games. If you're actually good, that's different.
America's Army is good for being free, but gets old.
I was good at Halo when it came out, but stopped playing so I'm probably super rusty.
I stopped playing Counter Strike because it's just too competitive for me. People are too good.
I play Team Fortress 2 the most right now because it's not based on graphics (like 99% of the crappy games coming out now), but gameplay. The gameplay is far better than most other games, and since it's team based you can't just be a tool and go around with the rocket launcher or whatever and singlehandedly kill everyone. It's very strategic (unlike the above games) but also very fast paced.
PS - I HATE STEAM! Ruins games. I just want to play a quick match of TF2, and it won't launch because the game "isn't currently available". Are you kidding me? My game that I bought from you just isn't available?