I have a few Korean shows I really really like. One is Stranger (or Forest of Secrets in Korean, not sure why they decided to go with such an unfitting name for english release), and the other is Signal
Stranger is a bit dense, but it's about a murder of a person acquainted with this capable prosecutor, who lacks regular emotional awareness due to a surgery to his brain. Despite his impairment, ultimately his goal is to make sure the truth comes to light. He teams up with a no-nonsense cop girl who is equally for justice as he is as well as equally capable, and they attempt to unravel this murder that is later discovered to be mired in a massive web of conspiracy that stretches to all sorts of unimaginable angles involving countless people and organizations.
Maybe from the description, it might not immediately grab at you, but it's incredibly well written and directed, at least season 1 is anyways. It's also just like a non-stop twist and turning mystery that constantly keeps you guessing. Also absolutely everything is never what it seems on the surface. A bunch of people with their own schemes and intentions constantly collide, and it really ends up feeling like a breathing world and case.
Signal is written by the same writer as Kingdom, but it's almost nothing like Kingdom. It's about a criminal profiler who finds a walkie-talkie that seems to communicate with a detective residing in the past but is declared missing in the present. Unfortunately they don't control when they can communicate, and it seems time is flowing for the both of them is completely different ways, making it difficult to always make the right decisions. Also any information that travels to the past seems to create time paradoxes in the present, changing the various present cases. However, using the information from the past, the present criminal profiler attempts to solve a bunch of cold cases with a detective woman, who was in a relationship with the detective in the past until he went missing.
Signal, unlike Stranger, is a series of various cases rather than being about one big mystery, but it does have an overarching mystery, obviously involving the detective in the past. Really entertaining shit