Damn.. thats a hard question to answer! Crysis was a lot of fun, and nothing can even come close to the graphics and the jungle atmosphere. Crysis was very enjoyable for me. On the other hand, STALKER took awhile to suck me in, but once it did I could not stop playing for a second. I seemed to have more freedom in STALKER and the story was something you had to figure out on your own, it was not just handed to you. I think the fact the STALKER was also centered around a real event made it that much more interesting. So its a very tought call, but I honestly think I would have to pick STALKER.
Imagine STALKER with Crysis graphics