STALKER isn't very "realistic" in the true sense. But it is fairly real - it drops you in the Zone around Chernobyl, full of other STALKERs, bandits, mercs, and mutants. You can free roam, find and upgrade guns and armor. It looks good and sounds good. Plenty of mods out there too. I'd recommend getting the Complete packs for the first 2 games if you buy them, or just get Call of Pripyat and there's some good mods like Ceano's mod enhances the Vanilla experience. There's mods that add new quests and characters. Some add better sound effects, others like AtmosFear make the weather effects much cooler. There's even an AI mod to make enemies smarter.
Cyrostasis is a very interesting game. It's not super awesome, but it's different. Basically you're exploring a nuclear powered ice breaker ship that got stranded. The crew is all gone and there's "zombies" that attack you - not really zombies but they're definitely not really human anymore either. It's a fairly lonely game, very story driven. Basically you're finding out what happened. There's also a wierd side tale that sort of ties into the end of the game. On top of that, an interesting mechanic in the game is that you can go into a corpse and relive it's last moments, changing the outcome and opening up the way for you in the present. There's also at least 3 different endings. Graphics are really really good in some aspects. Characters are a bit cartoonish, but the ice and frost looks great, and when it melts it's just so cool. Anyway, it's a cool and fun game. Some moments can get really intense/scary. (creeping through a kitchen full of mutant dogs who growl was rather hair raising for example)