I liked Red as a character, and how well it was described how paranoid someone had to be to survive an excursion into the Zone.
The thing that got me about Stalker was nothing made sense the first few hours you spent in the game. It actually took time to figure out what an anomaly was, and how to identify and work around them. You weren't given any warnings or clues. Just a minor visual cue that you most likely didn't learn to look for until you'd died a few times, unless you were lucky enough to see an animal run into one first and make the connection. It made combat so much more intense any time you entered a new area, because until you got really good at identifying anomalies, you didn't know how much you could dare to move around.
And the first underground lab didn't exactly feel alien, no... but the feeling of being haunted was the strongest I've ever felt. My first play through, I never figured out that you could kill the poltergeists. I shot at them, but it didn't look like it was effecting them, so I didn't unload enough to kill them. There was no indication that they were the things throwing objects at me, either. So I went through the entire lab not understanding anything that was going on. Balls of electricity and fire just wandering around. Random objects throwing themselves at me. It was also the first place you encounter Snorks, which are the most visually creepy monsters in the game. Then as you get towards the bottom, the pyrogeist starts fucking with your mind, randomly blurring and coloring your vision along with a haunting sound effect. And until you encounter the thing, you have no idea where that effect is coming from.
I don't think any other game has ever gripped me so hard.
I think maybe it's time I go download Lost Alpha. I waited long enough for that mod. Now that I'm reminiscing about Stalker again, I should go play it.