Worst: Any Mario game currently in existence. Seriously. They're pretty good games in terms of game play, but they make no flippin' sense.
Best: From what I've heard, (Not personal experience) heavy rain is really good. I'd have to say Assassin's Creed from my experience, though. The concept of being able to access your ancestors memories is amazing. Star Wars: Knights of the Old Republic is really good too, because of the plot twists involved.