If you have never played games from the Stalker series, you really should consider it. They are some of the finest FPS ever created. They do have some drawbacks but the story is top notch. It's kind of like Fallout with a better engine. The simulation of the weapons and body armor in particular is top-notch, much better than an average shooter sim.
Stalker: Shadow of Chernobyl - most expansive Single Player Campaign, lots of weapons and mutants
Stalker: Clear Skies - huge improvement in graphics,... See More
ability to join multiple factions, harder realism.
Stalker: Call of Pripyat - fewer areas, but larger maps and a ton of content and side missions. Also, the plotline really is folded together into a nice package. Graphics engine is vastly improved from the first two iterations. I can't imagine an FPS fan not enjoying each of these games for a couple run-throughs.