So I've heard ton's about STALKER but I've never played it... I'd make a request for one but I have no idea which one is the better of the 3... so I guess if your feeling generous and know which is better, it'd be much appreciated
In the game timeline it is first CS, then SoC and lastly CoP. In my opinion, this also from mediocre to best game.
Although there is some major spoiler in the end of CS and because the community rates it lower than the others, you might want to skip it.
Also if you have plenty of time there is SoC Lost Alpha which is free!! This a remake of the SoC including everything that was left out by the developers, plus some fan stuff, plus some modding, plus new graphics. Excellent for hardcore games who want to replay the SoC.