Empress Ki (2013)
Why? Great sageuk, great actors, intelligent, thrilling, tragic, dramatic and has you engaged from the start.
So many other great kdramas that are also my favorite, but as you ask me to recommend only 1 kdrama, I would have to say Empress Ki. It has a special place on my list, the only kdrama I would give 11 in score (instead of just 10).
Granted Empress Ki does not appeal to a lot of people, so when I recommend Empress Ki I do so revealing my own tastes and don't consider the person I am recommending to.
Usually when I recommend something to someone, or group, I have to consider that persons tastes. Does that person like comedy more than romance? Or action more than comedy? What kind of genres is that person into? I have a buddy that likes sci-fi, so I recommend sci-fi kdramas to him. To a group I generally recommend something that most can enjoy, like a contemporary drama/thriller.