Suggestion on underrated dramas

http://mydramalist.com/11582-heart-to-heart
http://mydramalist.com/471-9-end-2-outs

One Sunny Day (short and sweet)
I Need Romance 3
Queen In Hyun Man (the score isn't so low, but it doesn't get enough recognition, I found about this drama embarassingly late)
History of the Salaryman (as above)
Beethoven Virus
The King of Dramas 

I'm not sure if these are considered underrated or not, but here's some. Enjoy :)

You're All Surrounded (Lightweight police drama)
Queen Inhyun's Man (Almost no drama / Great love story)
Radiant Office (Currently airing!)
Thumping Spike (Short episodes / Sports / School)

Bonus: It's not Korean, but you'll like it. There's only 11 episodes for this drama. Definitely worth binge-watching!

Rich Man, Poor Woman (Japan / Office romance)

Mask (Drama, Melodrama, Psychological, Romance)
Queen of Ambition (Drama, Melodrama, Romance)
Doctor Stranger (Drama, Medical, Romance, Thriller)
Scholar Who Walks the Night (Historical, Manga, Melodrama, Mystery, Romance, Vampire)
Emergency Couple (Comedy, Drama, Medical, Romance)
Maids (Drama, Historical, Romance)
Cunning Single Lady (Comedy, Romance)

Mama
Can You Hear My Heart

God's Quizz and everything with the actor Ryu Deok Hwan is underatted ;_;

Beautiful Mind? One of my favorites! 

I don't know if "It's okay, that's love" is popular since I rarely heard or read any articles about it. Anyways, the script was really excellent and perfectly executed by the actors. 
Genre: Psychological, rom-com, medical, friendship, melodrama.

"Marriage, Not dating" was also an exceptional rom-com drama, I think its not a typical romance-drama since they present maturity and provocative scenes but still its real and lifelike.