Thief 2: the metal age. Best pure stealth game ever made, no exceptions. The art direction is wonderful, the voice acting good (great for it's time, and Garett and Victoria are still awesome) the world building is understated but utterly effective, the Antagonist is delicious, the mechanics are pure stealth without any of this 'oh woops I got caught, oh well, now it's just an action game!' nonsense that sours assassins creed (and to a lesser extent, Hitman - don't get me wrong, both excellent games that I love, but neither of them are really stealth games. Ones a parkour action game and the other is a puzzle game with guns.)
Yeah. Thief 2, because we praise the Master Builder for His Judgements.