Here are my recommendations:
1. Moon Lovers: Scarlet Heart Ryeo
2. The Princess' Man
3. Rebel: Thief Who Stole the People
4. Love in the Moonlight (more fictional than the others)
5. Jewel in the Palace (not so focused on romance, but it's very good)
6. I'm currently watching Dong Yi and enjoying it a lot. Already on ep 43/60, so I recommend this one as well :D
