I wouldn't say Stalker has a very good story, it's an excellent game, and well worth playing though, just the story is one of it's strongest aspects (possibly as a result of the translation).
The Witcher 2 is excellent and I wouldn't be put off it simply because it's an RPG, Baldur's Gate is another RPG yet to look at the two of them together they are completely different.
Portal 2 is excellent and the story is very compelling, in fact the game is really just a back-drop for the story.