Signal is an excellent mystery/thriller show. It doesn't have almost any forced romance/comedy scenes. One of my favorites. Weightlifting Fairy is a really good college romcom. It's a hot guy falling for a dorky girl show and pretty great one.
I'd recommend you BORDER (if you get bored you can always skip a few episodes, but, the finale is a must-watch!) Suspicious partner: nice comedy piece tonbi :)
Tunnel ( decent mystery, but the characters and their relationships make this) / king2hearts( for a badass female lead and fantastic secondary characters), and the korean version of liar game was quite interesting as well.