Ok I have only watched just over 20 Kdrama fully, so far this is my list:
Empress Ki - visually stunning, gripping storyline that keeps you interested for the whole 51 episodes, fleshed out main characters that evolve throughout the show, lovely acting and personally I love the romance in this(this has a male yandere, my favourite :-)).
Heirs - Classic school drama, poor girl, rich guy, an annoying parent, jealousy and a very prominent second lead guy.
Healer - well rounded with a bit of everything for most tastes, romance, action, mystery, you name it, this drama has it.
Cheese in the Trap - I think this teaches us about the complexities of human nature and that not everything is either black or white, but shades of grey.
Shopping King Louis - cutest and sweetest couple ever!