Honestly I don't have a drama that I can't stand, but the ones I enlist below are what I find overrated:
Healer - Everybody seems to love this but I just can't. I tried to like, I really did. But then when I was in my fourth time watching and disliking it again I thought to myself "nah, I don't like it and that's it."
Love O2O - This one is chinese and while I admit it's cute and kinda addicting it becomes boring pretty quickly, besides both leads are Mary Sues. I still wonder why it has such a high rating...
W - The beginning was good but then it became a mess, and not a funny one. Also, I don't know why but I couldn't stand the male lead.
Weightlifting Fairy Kim Bok Joo - I must admit I'll try to watch it again, but the first time I did I couldn't get past the second episode. It's slow in a boring way and it tries too hard to be funny
Secret Garden - Geez, this one. It was a guilty pleasure but still guilty. I couldn't stand the male lead's attitude towards the female, but I must be a masochist because I kept coming back. However, it's still pretty overrated.