The absolute best crime cdrama is Burning Ice . It is so mindblwoingly good, and so absolutely well done from a psychological aspect. It also is the previous work that the Light on Series is inspired on. Highly recommend it.(It also has a romance story)
Then for other recommendations I would 2nd The Long Night, which follows the same main character from Burning Ice, however this one is completely different. (no romance)
And also A Murderous Affair in Horizon Tower . This one is quite complex and has many many layers. Its well done (some degree romance too)
Additionally I recommend you The Kidnapping Game. It is also part of the Light on Series, but also has a absolute awesome romance story. Very well portrayed.
Burning River is also another awesome crime-drama that just recently finally got subbed. Its very gripping and amazingly done. (no romance)