I'll add mine.
Thief: The Dark Project & Thief II: The Metal Age
Jesus Christ. There hasn't been a game that has blown me away in the last 20 years like those two. First game to use shadows and sound as a stealth mechanic, and really the first game to use stealth period. Metal Gear Solid came out two months prior, but only used line of sight as a mechanic, not light and sound. The gameplay was so fluid, the voice acting was top notch, the graphics fit perfectly, the storyline was gripping, the cutscenes were fantastic, and the sound, dear god the sound. Even now I'll play those games with headphones on and get chills.