How similar is overworld section of the game to the Stalker games? Is it a huge overworld or is it broken up into chunks with crossing points?
Call of Prypiat is the best one just make sure you mod it out to make it even better...There is one mod that rules them all and I can't remember the name of it now sorry, a quick Google will find it though.
Have a look for the 'Complete' mods for the STALKER series of games. They don't change the game, just improve everything. After that try some of the wilder and more expansive mods.
My favourite of the series is still Shadow of Chernobyl, some of the underground sections are quite terrifying and will have you jump out of your skin. Call of Pripyat is a well polished game, but lacking a bit in atmosphere imho. Clear Sky is okay, and the swamps are fun, but it feels a bit of an also-ran compared to the others.