I personally prefer S.T.A.L.K.E.R. Metro 2033 is great if you want graphics and story, but I feel that S.T.A.L.K.E.R. does everything else better. I enjoy the combat, mechanics, RPG elements, settings, and level design a lot more than Metro 2033's. Don't get me wrong though, Metro 2033 is still a good game. Also, Shadow of Chernobyl has a lot of amazing mods that you could add to increase playtime, though not necessary (I've probably beaten it around 5 times with mods and 5 times without). I would get that first. If you like it, I would go ahead and get Call of Pripyat, most people prefer that but I don't simply because it felt like a lot of the challenge was gone. If you don't like Shadow of Chernobyl or have some extra cash, get Metro. They're both under 10 dollars anyways so you shouldn't have a problem getting both unless you're young.