In 1986, the world's worst nuclear disaster occurred at the Chernobyl power station. Soviet authorities established a 30km 'Exclusion Zone' around this nuclear wasteland, but in 2006 a second explosion rocked the stricken reactor, obliterating all living things and causing the Zone's boundaries to ripple outwards. From this epicenter came waves of mutated creatures, deadly radiation, and a strange, anomalous energy. The Zone was cordoned off by the military, who would shoot on sight anyone foolish enough to brave the horrors within.
One of the best games ever made.
Thus game is dripping it atmosphere, it oozes it. Furthermore, its one of very few games which can make me genuinely scared, it feels like you're constantly on an knife edge.
The game is open world and in a literal sense, it gives you the freedom to wander into the later zones if you want and, provided you're good enough, you wont just die. It doesn't suffer from terrible level scaling, though, it is harder when you reach later zones, much harder, but it also rewards skill.
Since I'm on moddb I probably ought to mention mods, there are a lot, and the community is still active. Mods can change this game into an even more atmospheric and tense game, they can make it into a full survival game where you find yourself scavenging for food, or, they can make it into a more action packed and story driven game. Lost Alpha is probably the best example of this; Lost Alpha changes stalker a lot, it changes it from an open world and atmospheric shooter into a story driven, and really quite linear, shooter in which you can backtrack.
Play vanilla first, before any mods, otherwise you will miss out because, however good they are, mods change a game and vanilla stalker has, for me, a special something which just cannot be beaten.
Doesn't matter how many games you play, S.T.A.L.K.E.R will always be one of those games you simply can't forget, even unistalling this game feels like a sin.
Mods just make the experience even better, It's not easy to put a bunch of mods together as it is with Fallout or other modern sandbox games but who needs 300 tiny mods added via a mod manager when you have The Zone and a single good mod taking care of it for you as you explore it deeper and deeper?