Stalker, especially Shadow of Chernobyl, is quite an experience the first time through, highly recommended. If you decide to try them, I would avoid modding it outside of using the Zone Reclamation Project for bug fixes. If you finish them and still want more, there's a standalone free play mod called Stalker Anomaly that updates the engine, gives it a huge visual overhaul and has its own slew of mods that can tweak it exactly to your liking.