I remember STALKER Shadow of Chernobyl vividly, with the excellent a-life somehow imbuing the whole game with a sense of 'living there' that few games have ever matched for me. The game was never the same twice - and every play-through always held at least one 'jump out of my skin' moment, along with odd moral dilemma where you could see a pack of dogs attacking friends/enemies/neutrals with your actions being your own.
The pacing was in parts almost willfully glacial, sometimes feeling more like a walking simulator than an FPS - albeit one with often sudden and very brutal deaths - with careful cache management often important. I had it on my PC at the same time as Crysis and didn't initially understand its appeal and yet, an o/s rebuild later, STALKER was reinstalled and Crysis binned.
Given it was the modding community that gave the STALKER games longevity, I hope the new games are mod-friendly, and here's hoping STALKER 2 keeps its eyes firmly on the low-key weird as there was often something deeply satisfying about getting what was needed without resorting to a head-on battle.