I can't think about a drama I hated so much it became the worst one. But I would say that my least favorites ones are:
Healer - I am not a big fan of action dramas
Fight For My Way - I like this type of drama but for some reason, I just felt bored the whole time
She Is 200 Years Old - I didn't like the acting and the characters.
My Sassy Girl - Boring
The Secret Message - I couldn't understand the plot and connect to the characters
The Fiery Priest - I wanted to like this one so badly but everything was boring
Refresh Man - First Taiwanese drama, binge-watched it and yet I felt bored and just wanted to sleep