I love this topic, I always discover other gems while reading!
9 End 2 Outs - one of the most underrated dramas in my opinion, but everything is great. The storyline developes in a natural way and its contents are quite original as well. Not everyday you find a drama where the leads start as friends and the transition isn't weird.
What's Up Fox? - one of my all time favorites. It's refreshing, the leads are both headstrong and with their own principles. The story develops nicely, not too much drama that would get on your nerves. Also the chemistry between Go Hyun Jung and Chun Jung Myung makes sparks fly!
The Vigilantes in Masks - if you are craving a wuxia look no further and start watching this. I recommend this drama every chance I get, it's amazing. I love all the characters and how different they are from each other. It has everything, form action to romance and depth.
Divorce Lawyer in Love - this drama really didn't get much love when it aired. But in reality is a better romcom and drama then other more popular ones.
Jiu - Another really well hidden gem. It has suspense, action and all wrapped in 9 episodes. The female characters are strong, with their own characteristics. It had me on edge for the entire course!
Maids - again an overlooked drama. It was lost in the sea of other historical pieces and because there were no very popular actors in it none give it a chance. But I am sure you will not regret if you watch it. I loved the female lead, how she developed through out the story and how the environment changed her!
Mirai Nikki - what is so good about this drama is that it's so short. But that doesn't mean that it isn't complex, think about it as drinking a strong espresso in one go. Not many people watched it and I find that to be a pity. So don't wait any longer and give it a try!