100% the best drama of 2019 for me is The Story of Ming Lan. It technically started airing in 2018 but it was on Christmas day so it was the very end. It blew every other drama I've seen out of the water. My new favorite drama of all time.
Some honorable mentions that I've loved this year:
Arthdal Chronicles - hands down the best drama of 2019! It has everything; talented cast, intricate and complex storyline & worldbuilding, dynamic and interesting characters, as well as top-notch cinematography.
I've only watched about ten or so from 2019 since I only started watching Asian dramas about a year ago and have plenty of content to keep me occupied from previous years. Still, my top drama right now is The Untamed.