I know that there are plenty of good K-dramas in the recent years and there will definitely be more.
But what is the cream of the crop, tip of the top, timeless masterpiece K-drama that you will recommend.
To set a relatively low bar so that more people can participate, make sure your K-dramas watched count are at least 50+. Also make sure you are at least somehow following and up to date with the K-drama scene as standarads are constantly rising.

As of October 2021, my pick is still My Mister (2018). 

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.

Ditto on My Mister.  I've seen over 100 K-dramas, and all of the top-rated ones,  but I can't think of any I've seen that even come close to My Mister.    Moreover,   nothing in English was as good  either, except perhaps season four of The Wire.  But really,  not even that (and that series was stupendous).