Thief II and Thief TDP/Gold are close in that regard but in my eyes T2 definitely takes the spot. LGS focused on what made the first Thief game good; urban stealth. T2 almost exclusively focuses on thieving with the exception of two levels where as one falls into the supernatural spectrum and the other is a (good) IKEA simulator. T2 had a rocky development although it's really only visible in the Casing the Joint and Masks missions, with the former being arguably the worst mission in the game. Would definitely recommend this one, not to mention the T2X: Shadows of The Metal Age fan campaign and countless other fan missions.
Thief: Gold (along with TMA) is definitely one of the best stealth games that you can play. The game has aged well (except for things such as graphics, but graphics don't define a game for me). Gameplay is immersive, voice acting of characters is silly but good and the story is interesting. The level design is good for the most part, however it is an improvement for sure in the sequel. Not to mention, unlike the Steam version you don't need to jump through any hoops to get it working properly.
