Even without all the voice acting, fancy graphics, humor and story, Portal 1 and 2 is one of the best puzzle game/series hands down.
The portal concept is one of the best game play innovations to come around lately. I'd go bananas over a futuristic themed FPS that incorporated this concept, and I don't usually play FPS.
Now add all the voice acting, etc... and it's clearly hovering around the top. And it has an excellent, simple control scheme.