STALKER: Shadow of Chernobyl.
It's quite a bit different than 90% of all the other FPSes out there, enough so that it takes some getting used to. Most players will feel kinda "lost" first getting into it. You're not forced to follow the games storyline in any specific order, nor is there any time frame involved, either.
Besides, the modding community is extremelly active, and there's tons of awesome mods available for it, too - keeping the game alive, fresh and well.
There's a lot of myths about the game being buggy as hell - but once patched there aren't really any major issues. The AI tends to occasionally get glitchy, but the AI is quite complex - you won't find many other games on par in terms of the amount of stuff that goes on "behind the scenes."
Metro is good, too - but extremelly linear. You'll get more enjoyment in the long-term from STALKER. I'd start with either SoC or Call of Pripyat, first.