Not necessary "worst dramas" … more like too much hype … my list:  Rooftop Prince, Warm and Cozy, The Heirs … to name just three.
Boys Over Flowers.. It's like a hatelove drama, you either love it or hate it. In my opinion, they ruined the whole damn concept. Hana Yori Dango is way better.
Cheongdamdong Alice, I cringe just to think of it
Birth of a Beauty, I REALLY liked the beginning, but the ending was ridiculous
1. Full House Take 2
2. My Lovely Girl
3. Mary Stayed Out All Night
Passionate Love
I Am Sam
Best Wedding 
1. Doctor Stranger - This drama highly disappointed me, if it wasn't for Lee Jong Suk I would have dropped it. It didn't know what it wanted to be and the plot wasn't consistent.
2. IRIS - That ending clenched the hammer on this one.
3. You're Beautiful - Female lead was too annoying for me, best thing about this drama was Jeremy.
1. Playful Kiss
2. The Heirs
3. Meteor Garden 2
Worst of the worse:

So bad I don't think I will ever to get to see awful these are =(

Ireland= Omg .....dare I even say the words. Brother and Sister hmmmm
Ice Girl=Horrible story line ....frozen  than brought back later. ..yikes who are you going back to
Manny = only because it's  the only drama so far  that i failed to finish simply because i just couldn't. 
It's fascinating that the significant number of my favorite dramas is on this list.The Heartstrings, BOF, Queen In Hyun's Man, Goong, The Greatest Love, Secret Garden, Master's Sun -  so just shoot me, but I simply adore this shows. However, everyone's drama taste is unique - that's for sure...

The worst - top 3

Korean
Panda and Hedgehog
Nail Shop Paris
Birth of a Beauty

Taiwanese
Romantic Princess
Skip Beat
Bull Fighting

 
1. boys over flowers 2.playful kiss 3. bride of the century
1- SPY
2- Blood
3- Doctor Stranger
I'll just mention two: Hana Yori Dango & Boys Over Flowers.
The story made me dislike Matsumoto Jun/Lee Min Ho because of the character they played. Although I liked/didn't know them previously, the arrogance of Domyoji Tsukasa/Goo Joon Pyo & how much I hated that character is all I could think of looking at their faces. Thankfully, Lee Min Ho redeemed himself as I just watched Faith and absolutely loved him in there. 
- Boys over flower 
- To the beautiful you
- Skip beat 
Top korean :

Prime minister and I

My lovely girl

Haeundae lovers
top tw drama :

Romantic princess

Bull fighting

Déjà vu
between dramas that I've watched:

1- Heartstring 

2- My girlfriend is a Gumiho

3- The winter the wind blows