While this one is the best, the other two STALKER games are extremely good. Don't expect to be able to run through this game, it will take patience, skill and planning to traverse the amazing world here.
While some may call it the weakest of the series, this game is simply superb, it can only be overshadowed by CoP or SOC. This game has a faction reputation system, very fun to integrate.
This game has it all:
Creepy Atmosphere
Intense situations
Realistic combat
Multiple endings
Side Quests
By far, the best FPS experience I've ever had (along with CS and CoP)