Let me start off by saying that I'm not a big fan of First Person Shooters. The tend to get a little bit nauseating, and I started my "video game career" with platformers, so I prefer to see my character in relation to his/her/it's environment.
I do, however, really enjoy playing Call of Duty 4: Modern Warfare. I don't own any of the newer games (I can't justify spending the money because I hardly play the games I do own as it is) but owning COD4 is great because if I want to pick up a quick game and shoot some enemies, I can do so.
I bought the game shortly after it came out and I beat the campaign mode in two weeks or so. I then played a bit of online (I believe it was the most popular online game at the time) and although I wasn't super-great, I could still hold my own. I like that the maps and ammunitions you can choose allow anyone to take advantage of his or her strengths.
I wonder what the future is for the Call of Duty games. Obviously Activision has cemented a multi-platform series. I hope to see some more historically set games in the mix, but they'll inevitably make MW3 and MW4 as well. I hope they can jazz it up with some creativity.