Best STALKER game?

SHOC

Shadow of Chernobyl :)

cyprit

clear sky

Any is the different. Shadow of Chernobyl - (My best, i played only this and Clear Sky, but i bought Call of Pripyat today ;]) you have there a X16, X18 etc. (labs.) - scariest locations in the game, you have there only 2 fractions who you can join. In "Clear Sky" you have 3 and there's is good system for the fractions battle but the - for me - worse mechanics and storyline.

shadow of chernobyl has the best story

clear sky has a faction war system that doesn't work properly

call of pripyat has the best side quests

Call of Pripyat

Pripyat is the top choice. Although Chernobyl was first chronologically. Avoid Clear Skies.

Go with Pripyat if you can only get one.

Chernobyl was awesome, full of atmosphere, hard but not frustrating, freakin' awesome story... I loved it, I think it was the best one.