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's a fairly old mod that I'd actually forgotten I had until I saw it mentioned at GSC today, It was put together by Mechanic and a small group of Russian modders. If you use the variation mod here on MOD DB this is easily merged with it allowing your third person to show the correct helmet for the suit your wearing.