Concept-wise and in the size and pace of its engagements, Monster Hunter Ltd. Parts 1 & 2 (#073) is more like a big budget game than most of what I've played on this list so far - STALKER: Shadow of Chernobyl specifically comes to mind. A detailed real world setting twisted into uncanny horror, built with a high level of polish and an exceptional eye for texturing. Towards the end, you encounter one of the scariest things I've ever seen in a Doom level.