Stuff like You're beautiful (with all it's remake : Fabulous Boy, Ikemen Desu Ne), Heartstrings, Coffee Prince and Dream High, I watched it when I was 11-14 years old so I kinda enjoyed them. So I can't really say there are the worst . Even if I must admit that I wouldn't be able to watch any of those in the present days  ( I tried it was a bad Idea). All of those are pretty much the same old romance story with just a change of scenery ^^ After a few of those it's understandable to be bored.
But stuff like Fated to love you and Playfull kiss, I just can't accept that. In Fated to love you, I excepted something mature since the main leads were adult and not high school student. I got something below elementary school student romance. Below !!! Everything was so ridiculous. But the worst was the misogynism or sexism or whatever you call it. Yeah sure there was a bit of that in stuff like You're Beautiful but in this one it reached a whole other level.
Let's just not talk about Playful kiss (How could this have so much adaptation ?).

Another deception was You Who Came from the Stars. I watched half of the first episode and dropped it. The male lead was unoriginal (if they actually taught that the alien thing made him a little bit original, they were wrong) but I like cold-hecarted bastard. When the female appeared, I couldn't believe "that" was a female lead. But it seems that it's the case. For the first time in my life, I tough weak-willed, ridiculously naive and cry-baby heroine weren't so bad after all.

 
Heartstrings, Bubblegum, Bride of the Century
I'd have to say Pretty Man as first on the list. Omg that was such a pain. I kept telling myself to try to be patient and watch it and that it might get better but nah. I couldn't. It was THAT bad. Second place is Please Take Care Of The Young Lady/My Fair Lady. Another one I dropped. Third is You're Beautiful (I'm sorry). Though I did finish that, it was actually worse than some others I've dropped
I would say Full House which a lot of people say that actually liked it, I honestly forced myself to watch it, it just felt like it dragged so much...
Also Drama Go Go Go, which I liked the beginning but it got dragged to much, yet again.
The Most Unbearable to watch:
Pretty Boy/Man , Marry Him If You Dare , Flower Boy Next Door
The Great Seer is the only drama that I sincerely hate with a passion. I actually watched the whole thing just to see if it got better. But it didn't. It only got worse and worse. I honestly regret finishing it. Definitely a waste of time for me. Sorry, I do sound harsh, but like I said, I hate it with a *passion* xD. 
The very beginning was good though. It seemed so promising! Oh, how misleading that first episode was...xD. 
orange marmalade
heirs
high society
cheese in the trap
I haven't seen anything I'd actually classify as bad, but Hana Yori Dango legitimately made me depressed to watch due to the main relationship (I stuck it out because I don't like to leave things unfinished, but probably should've quit watching at some point), and From Five to Nine was problematic for me, as well.
1) Missing you:  I'm surprised the whole of the Korean peninsula plus half of China wasn't flooded for years after all the crying in this.  I watched the whole thing and i'd like those hours back.

2) Thorn Birds.  A tragic story that each and every character deserved after making me not care about any of them.  Just urgh.  what horrible people.  I dropped this at episode 6

3) Playful Kiss/Goong  Third position is tied, these are equally bad and the unhealthy power balance in the relationships is disgusting.  Dropped both of these early on. 
Doctor Stranger - it kept getting more and more ridiculous and the female lead was so bad at acting..
My Lovely Girl - Story was going nooooowhere and who the hell falls in love with your dead girlfriend's sister??
1. I need romance 3  2. Heartstrings  3.Cheese in the trap
another miss oh, Dalja's Spring, Cheese in the Trap -_-
fashion king
when a man's in love
easy fortune happy life
Goong
Hana Kimi
the heirs
KDRAMA:
Heartstrings
Pride and Prejudice
Haeundae Lovers
The Heirs

TAIWANESE:
Smiling Pasta