On another forum, I mentioned that the Ravenholme part of Half Life 2 didn't strike me as especially scary. Another person said it was the scariest thing ever. I said that, out of recent games I've played, the snork infested abandoned labs in Stalker: SoC and parts of Metro 2033 gave me more heebie-jeebies than Ravenholme did. They said I was insane and we agreed to resolve it with pistols at dawn. That I'm posting now should let you know how that ended.
I don't contend that any of those examples are "scariest ever!" but let's hear some well done scary games (or just parts of games). I'm sure Amnesia will be mentioned soon enough -- I have it but still haven't played it.
Define "scary" however you want. I've never played a game that made me wet myself or gave me nightmares but I've gone through parts that made me feel jumpy and uneasy while I went through them.