That Winter the Wind Blows
I'm going to have to choose between Reply 1997 or You're Beautiful here, but since it's only choosing one...I'm going with my first love in kdramas world: You're Beautiful. However Reply 1997 is pretty close here!
My name is Kim Sam soon ( it was the first korean drama I watched)
Innocent Man
Sorry I love you
Cruel City ( this drama legit changed my life LOL)

It's hard to pick one favorite if you have seen so many dramas loooll
I'm not sure I can answer that question but I will try !

1) Iljimae
2) Tree With Deppe Roots
3) Time Between Dog & Wolf
4) The Princess'Man
5) Warrior Baek Dong Soo

& then : Empress Ki , Bad Guy , Sweet 18 , Queen Seon Deok , History of the Salaryman , Sorry I Love you , The Man From The Stars , Delightful Girl etc.
The Greatest Love

It's a RomCom drama but with sad story