1. One that really stuck with me was The Long Walk by Stephen King writing as Richard Bachman.
I think just because it’s rather intensely focused on survival as a competition (100 young men, 18 years old, walking continuously until there’s only one left. The others will die of exhaustion or breaking rules, one of which is a minimum speed limit that must constantly be maintained) and the psychological effects of that on the contestants.
2. Gerald’s Game is a great book, but it also made me want to throw up.
3. House of Leaves by Mark Z. Danielewski is a long read but a solid contender to be considered one of the most unsettling, dark novels I’ve read.