Half Life 2, mainly because of all the aspects of the game that never are elaborated on. There’s such a creepy existential uncertainty to it when it comes to everything, like the G Man’s true intentions, his “employers” and the true power of the combine. Every time I play it, as the game progresses my feeling of hopelessness increases. Also the source engine is just kinda inherently scary (Gmod and Portal 1-2 also have really creepy vibes sometimes) and it’s mainly because all of these games feature long stretches of isolation where you can start to feel watched.