ago, Deus Ex. The story was immense (almost to the point of ridiculous and the literary references were pretty obscure for anyone not a total science fiction, poetry, and conspiracy theory nut) and the player can decide how to play it, from totally non-lethal to full-on Rambo. It was a great blend of FPS and RPG. The maps were very open and the thought and detail included were amazing for the day.
After that was the straight on FPS Call of Duty. it did suffer (IMO) from being a strict tunnel walk-through-the-story-and-shoot-everything, with some serious faults in the American's weapons (200 yd one-shot kills with a Thompson are common), but overall they went to great lengths to create as much realism as possible in the context of a video game. The on line game against other real people was amazingly hard. If you want to shoot somebody on the run from 300 yards have to lead them and account for drop, wind, etc.
In both cases, once they were successful they were sold to giant corporations that proceeded to axe everything that made them great and push out horrible sequels. The follow up to Deus Ex was a total disaster while COD was slowly killed with each imitation being just a little worse than the last.
Wow, how did this turn into a rant? Sorry. I will check into S.T.A.L.K.E.R., hopefully it is another game created by people that love games more than the money they might bring.