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.
An absolute monster of a game, both time & environment size wise. By far the best part, is the friendly-to-modding base of the game, and the vast array of mods & mod tools available.
S.T.A.L.K.E.R. SHoC rates highly by itself (I won't be so brash as to say it's the pinnacle, but only because I've not played every single game to compare & contrast against), but after adding the mods/mod tools into the equation, it stands head & shoulders above anything ever played before, or since by me. Whilst it has never been perfect, it is/was such an ambitious undertaking, that one can't help but admire it. Even if it did release gutted to it's very core.
It's certainly not for the short attention span tl;dr type. More involved & immersion intensive a game though, one cannot find.