It's a tough question. There are a ton of excellent survival horror games! However, if I had to choose I would say Silent Hill 2 (2001).
Silent Hill 2 recognizes its titular environment as a psychic space as much as a physical one. Like Resident Evil 2 (1998), it was my very first Silent Hill game, so I am very fond of it.
A close runner-up title for me would have to be The Evil Within (2014).
Reasons to be interested in this survival horror title can be boiled down to just two words: Shinji Mikami, the designer responsible for original Resident Evil games and the survival horror genre.
What about you? What is your favorite survival horror game? Don't you dare say Resident Evil 2!