"It's Okay to Not Be Okay " has me finally and completely hooked into K-Drama. Loved that show, the chemistry of the leads, the hilarious humor, the fantastical elements, the creativity, the murder mystery and of course all the charming quirks of the characters. Continued with "W" and "My Holo Love" and liked both very much. 

At the moment watching "Bring It On, Ghost" and "Hotel del Luna". 

But where to go from there? What are "must watch" shows for newcomers to the genre and why?

My watchlist: https://mydramalist.com/dramalist/8357349

I'm recommending some dramas that are semi-popular, but I still recommend to beginners, besides Goblin which is super popular I'd say.

  • My Mister - I saw it's on your watchlist.  I highly recommend it, it's very different from all the other dramas I've watched in tone and characters.  I ended up binge watching it in a few days.  It has by far my favorite leads of all time.  Though, fair-warning, this is a very depressing drama.
  • Be Melodramatic - This drama is a mix of slice of life, comedy, romance, and melodrama.  It has great female leads and I love the supporting cast as well .   And if you end up liking this drama then I recommend Because This is My First Life, the only difference between them I'd say is Because This is My First Life is more romance-centric.
  • Are You Human Too? - Since you liked My Holo Love, I recommend this one!  They are very similar in plot and I'm currently watching it.  One thing I'd say is that at the beginning the female lead isn't that likeable, but if you keep on watching she gets better.
  • Splash Splash Love - This drama was one of my first dramas and it's a really good introduction to dramas I'd say since it's only 2 episodes long yet so fantastic.
  • Goblin - Also on your watchlist.  This is a incredibly unique drama, it has fantastic cinematography and it has the most memorable OST (it's my favorite soundtrack in a drama to date).

Fantasy-mystery-romance genre dramas:

  • Goblin
  • Chicago Typewriter
  • Mystery Pop Up Bar
  • Oh My Ghostess
  • Pinocchio
  • While You were Sleeping
  • Queen InHyuns Man
  • Sensory Couple
  • Set in historical period:
    • Arang and the magistrate
    • Scholar who walks at night
  • Meet me at 1006 (taiwan)

I agree with  Goblin. 

Since you said you are a new comer + you seem to enjoy the supernatural + romance Goblin is the best of the best for this.

It has a great story, very funny moments, cute bromance as well, some good teary moments, also great mystery. Its truly a gem of a drama you will not regret watching next ;) 

Goblin
Strong woman Do Bong Soon

Descendants of the sun

thx for all the suggestions :)