Call of Pripyat, but play Shadow of Chernobyl first, you won't understand much if you don't. I also think some of the moments in Shadow of Chernobyl were MUCH more intense.
shadow of chernobyl had more setpieces, and said setpieces were absolutely amazing
i'm loving call of pripyat, but the underground levels aren't as good as soc's
it's a lot more open and freeform, though