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.
Link for a new winter mod, I've not found much info yet but Cromm Cruac put up a vid link and a rough translation of the concept at GSC. It has an original storyline that sounds promising.
Background: The Zone has been covered by a huge anomaly that consumes heat. In some places the temperature can fall as low as -60C. Most of the settlements has been evacuated and left abandoned. Meteorogical data suggest that the center of the anomaly appears to be at the CNPP. Our mission playing as an SBU agent will be to determine the origin of the anomaly.