Get Farcry or Half Life 2.
I thought Farcry was a better "game" as far as the story and gameplay goes, but Half Life 2 definitely has the most realistic environment/characters of any game to date, so they are both worth playing. I had a weird problem with my Farcry though, the sound was delayed when I shot. I click shoot and I would see the animation and everything but my sound wouldn't come until a sec. later. This is probably fixed in a patch, or just a weird freak thing with my computer, but just thought I'd mention it.
And lots of people rave about CS and CS:S but really they're not that great. As was mentioned before the community is horrible. A bunch of people who swear and hack everytime they get killed. Importing porno sprays and stuff....it's pretty stupid.