The Witcher.
Simply because the game mechanics are tailored around the books' distinct atmosphere. Not the other way around. Visually, story and gameplay-wise it perfectly recreates the life of a prone to philosophy, genetically modified monster slayer and the world surrounding him.
The main problems were that the game was buggy and unpolished quite a bit above the accepted level, but CD Projekt handled it marvelously with the free Enhanced Edition.
To some it may not look like much, but it struck me as a great game when i started reading the books and found myself in familiar waters, reading about characters and places i've already met and seen, that made an impression on me without relying on their source material origins. They were a one to one adaption in every tangible way, not a mere bland copy.
The gameplay too perfectly incorporated in the story. It's one of the few RPG that force you to roleplay, the world is a whole and you interact with it in different ways, not just by killing things and then selling things. Combat with it's focus on preparation, knowledge of your enemy and positioning is also a great take from the novels.
The game might as well have been the source for the books, i'd have hardly noticed.
