Probably Dark Souls 1. We're so far from the original release of the game now that I actually cannot imagine playing it and getting my head kicked in, it's too easy now, but there definitely was a time when it was an enigmatic and dangerous world... it's a distant memory that I want to re-experience.
Also Silent Hill 2, because I played it as a kid and didn't really get the full weight of the experience even though I still loved it.