I was lucky to ask my friend about Portal right in the two week window when it was free to promote the Mac release of Source games. So I got that for free. And then I got TF2 for free. Both are fun, but Portal has no re-play value.

As for Call of Duty, I only have 1, United Offensive, and 2. 1 and UO are the only Call of Duty games that concentrated on the single-player, and therefore have it the best. 2 is alright, but anything later ( I tried CoD 4) is just a joke in single-payer, and I didn't get those. And I absolutely hate the texture/mesh clipping in the Cod games after UO. 2 is a serious offender, where things will pop up five feet from you. So I like the older graphics better.
An FPS that I really like but is not for everyone is America's Army. Multiplayer only, with a low player-base (for Linux and Mac), but its really fun. You only get one life per round, and ballistics are real. So you don't rush like in CoD. In fact, AA has rules that determine what kind of camping you can do instead of banning campers.
Similar to AA is Rainbow Six. Think if RS as AA, but with an excellent single-player campaign. At least, RS 1-3. After Raven Shield, you might as well get CoD.
I also play Battlefield 2142 sometimes. It's unrealistic as heck (same as CoD), but it's future setting makes it easier for me to not scream "It doesn't work that way!".
But at the end of the day, I prefer realism over acrade-ish styles, and therefore for me, AA and RS are more fun than CoD. TF2 is fun because it doesn't pretend to be serious unlike CoD and Battlefield. And people don't claim to be military experts after playing TF2.