What’s the best psychological thriller you’ve read on KU?

I’m in need of a distraction and the best kind of distraction is a good psychological thriller. So what’s the best you’ve read on KU? I’ve read some work from John Marrs, Freida McFadden etc. and they were okay. What I’m looking for is
1) hopefully good writing
2) dark (darker the better)
3) a good story (difficult to guess the end, nice character development etc.) with less potholes.

Thank you!!