The second game in the series, but the first chronologically (being a prequel), this game is by far the most underrated of the series. 9/10, if you ask a S.T.A.L.K.E.R's least favorite game in the series... it's this one, it gets a lot of hate for the fact that on release it was very unoptimized and buggy and there really wasn't any workaround for it. But now, of course, like any other game in the series, a ton of mods are available for us to choose from and almost all of them include bug fixes and optimization features. Some of them stop there, upgrading the graphics a little, optimizing the game for you, and fixing common bugs and issues. Others, however, expand on the open faction system that this prequel provides. Unlike in Shadow of Chernobyl, this game allows you to interact openly and work with the factions outside of their main quest lines, going as far as joining them after reaching a certain status; and that's not just Duty and Freedom, that's almost any faction in the game.
The story, while not expanding too much on the events its predecessor, provide a little insight into what was going on in Shadow of Chernobyl and why it was happening, as well as revealing to you (Scar) information that Strelok did not know at the time. Don't get me wrong, Strelok is a cool guy, but Scar is probably my favorite character of the series. Either way, don't ignore this game because of earlier setbacks and whatnot, with the mods you have available now you can experience the game in almost a completely vanilla state (if you so wish) with all of the bugs and unoptimized mess fixed and cleaned up.
Again, an excellent series; the story, atmosphere, characters, and mod variety all make it worth its price. Tons of replayability to be had.