One underrated thriller that I absolutely love is "The Chaser" from 2008. It's a South Korean film that delivers such a relentless pace and tension, blending psychological suspense with some hard-hitting action. The way it portrays the cat-and-mouse chase between the detective and the serial killer is masterful. I'm a huge fan of how South Korea has pushed the boundaries of the thriller genre, just like they do in their action cinema.
Also, keep an eye out for "The Guest," another gem that combines supernatural elements with a gripping narrative. I think both films create an atmosphere that really keeps you on the edge of your seat. Have you seen either of those? What specific elements do you look for in a thriller?