Here are 5 kdramas that I personally think were perfectly created. They are in no particular order btw. :)
Black - I'm a huge crime, thriller, supernatural and romance fan, finding a drama that mixed the four was an absolute blessing! The drama concentrates more on the mystery but you can sense chemistry between the leads. Its a story about a cold grim reaper and a girl who can see ghosts.
Psychopath Diary - This drama doesn't feature any romance but i decided to give it a try due to the interesting plot and for Yoon Shi Yoon's acting. This dramas plot was so so good and Shi Yoon's acting made it so much better. A weak man accidentally sees a serial killer while trying to commit suicide, he takes the serial killers book but loses his memory. When he wakes up he finds the book, reads it and begins to believe that he is the serial killer. This drama was hilarious but also very dark, highly recommend :)
Love Affairs in the Afternoon - Dramas like this are definitely not for everyone. But I loved the secretive, sweet romance between them. An added bonus for this drama was the great visuals and OST! But if you are not a fan of infidelity/forbidden romance dramas I wouldn't recommend this.
One Spring Night - This drama is liked by a lot of people, I had always thought it wasn't my type of drama but decided to give it a try and it ended up being one of my favorites! The romance between them was so pure and adorable. I loved the even though the ML wanted to find a lover, he would always put his son first, luckily for him the FL was very open about him having a son and was very willing to consider him her own :). They go through a lot of struggles together but the outcome is beautiful :) Bonus: Great visuals!
W - Such a creative plot! I loved the acting from both leads along with the secondary characters,.If you like plots about star-crossed lovers I think you may enjoy this. Their romance progressed quite quickly but it was clear that they wanted to protect each other :)