I wanted to know , what dramas ,got you hooked from episode 1 and you just couldn't stop watching until the end ? Will give all a try if not seen yet ! For me I have many : but ones that comes to mind are MR. Brain , and Two weeks
Did you watch Gaksital? ^^ I was totally addicted to it.
Have not watched it , will give it a watch though !
Vampire prosecutor 1 and 2 - just amazing! I was so mad that I had to go out for classess when I started watching it and couldn't concentrate on anything else Ten Master's sun 49 days Rooftop prince Kurosagi Unfair You who came from the stars - I know it still hadn't finish but every episode is so amazing that it's even hard to describe Emergency couple - it's also very addicting
Iris Sign Reply 1997 : D
I hope you will like it too :)
There's been plenty but I'll go through a few that were surprisingly good.

Let's Eat - I didn't really know any of the cast. I don't like food dramas. The trailers were... meh. But since it was about single people and neighbours, I was intrigued and had to check it out... And so, I loved it from the very first episode.
Once Upon A Time in Saengchori - Again, didn't know the actors and wasn't even too sure how that episode 1 made it's way to my screen. But it was so good!
White Christmas - I heard rumours that a certain drama special was gathering a bit of cult reputation. But a school for genious kids? Sounds dull. But after I gave it a try? Woah.
She is WOW! - Weird plot summary but decided to try it out. And suddenly I was hooked.
missing u it's a different of all dramas i had seen .u can't stop watching.i like many dramas but until now i didn't watch drama that has all that surprises and story as missing u ,ahhhh i'm sure u'll love it
I'll strictly recommend the ones that I marathoned immediately after the first episode - these are the shows that I have probably watched in the least amount of time between episodes, so you could say they are addictive:

KDRAMAS:
Shut Up: Flower Boy Band
Missing You
I Hear Your Voice

JDRAMAS:
Orange Days
Kingyo Club

TWDRAMAS:
MARS
i miss you and the moon that embraces the sun specially other show that got me very interested are Q10 , shut up flower boy band , kingyo club and a gentleman's dignity
49 Days and Queen InHyun's Man
Gu Family Book, Don't Look Back: The Legend of Orpheus and You who came from the Stars
bridal mask is awesome 10/10 i can hear your voice 9.5/10 ghost
For me there have been a few of them.

Kdrama: Coffee Prince, and You're Beautiful.
Jdrama: Liar Game, and Bloody Monday.

I was pleasantly surprised by the Kdrama - Coffee Prince was the first Kdrama I watched, and You're Beautiful wasn't my usual thing.