For me you can't beat STALKER. I have tried pretty much every game out there but have found none that put me where this game does. I start playing and get lost in a world that can be both beautiful and terrifying. I remember my first visit to the Yantar lab in SHOC. It was in the early hours of the morning, I was using headphones while sneaking around the lab. The atmosphere had me on the edge of my seat. Ten seconds later something totally unexpected happened and put my heart in my mouth. I paused the game, shaking like a little child and from that moment on for me it has been the stick by which all games are measured. And sadly they just don't measure up. STALKER is a thousand games in one. Set in a constantly changing, evolving world. The only time any two playthroughs are the same is when you hit that ENTER THE ZONE button at the start of the game.
It may not really be a spoiler but some will think so, If you don't want a small but major grief preventing pointer stop reading NOW... There are tonnes of new quests added to SGM for this mod and it will save you a lot of hassle in the long run if you DO NOT sell any artifacts or mutant bodyparts. Just save up all you find of both as there will be times in the game where you could spend ages waiting around if you haven't saved what you've previously found.