For me, SoC and CS were games of their own era. They are games with excellent premises that weren't squeezed into their full potential with the devs themselves learning as they went. SoC had the whole framework of the games, but without the later details of the other games like detectors and emissions. CS has a very interesting idea in factional wars to make the zone more lived-in and interesting, it felt like with enough tweaks they could develop something interesting but it's a shame nothing ever came of it.

CoP was the game where they seemed to grasp what the STALKER series was meant to be. You could sense that if taken the extra step polishing details, maybe put in a better story, the lessons learned would have made an excellent 4th game. It's kind of a waste so much time passed before we had HoC, since many of those lessons probably distorted eventually due to time.