I'm sucker for well written antagonist. well even badly written ones i love the dramatic smile at the camera as the music gets intense, but i prefer for them to have a motive rather than just because they be a evil gangster lol

With as little spoilers as possible i'd love to find more dramas with great villains. i've seen finished alot of jdramas but only 8 kdramas.

so far my favourite villain is min joon guk from i hear your voice, i love his story and they gave a large of focus of why he was doing what he was doing

The best villain in k-dramaland is for me without a doubt Mishil from Queen Seon Duk. Not sure if you want to commit to a 60+ ep drama, but imo her performance alone makes it worth watching! Two other dramas with great antagonists are The Chaser and A Man's Story.
seeing over 50 episodes for dramas does put me off a little. but i if i really get into the story the amount of episodes shouldn't really be a problem , it will just take me a 100 years to finish haha
Mishil from Queen Seon Deok definitely 

don't be scared of its length cuz it's like one of the best drama of alltime, a masterpiece . You won't get bored even for a second 
Danashri from Empress ki! shes one of my favourite drama characters and even though she was so evil i still rooted for her
empress ki and innocent man
Interesting that you are a sucker for the antagonist :D Maybe you want to check out anything Shin Sung-Rok who excels at the villain role. He was excellent in Liar Game which was a very well done drama. He was also in My Love from the Star. I didn't like this drama much but he was a creepy antagonist in that one. Another good antagonist story is Healer. Park Sang-Won was a worthy antagonist with an interesting tale interwoven into this excellent drama. Give it a try! Hope you enjoy these recommendations.
1. I second Min Joon Kook in I hear your voice
2. *spoiler* Yoon Tae Joo in Hyde, Jekyll and me
3. for pure comedic value Gi Cheol in Faith
I second Shin Sung Rok. He was amazing in Liar Game and You Who Came From the Stars.

Also: White Christmas, Last Friends, The Quiz Show 2, Ningen Shikkaku, God's Gift - 14 Days (there are many here)
Seconding A Man's Story.  (Chae. Do. Woo.)

Also, how has no one mentioned Shunji from Bridal Mask?  Basically my two favorite dramas, and definitely my two favorite antagonists.  And those two things may be related.
Mishil from Queen Seon Deok and Chae Do Woo from A Man's Story. And the stepmother from Shining Inheritance.
@ zeamays: I think nobody mentioned Shunji, because he isn't (in the first place), a villain/antagonist. I think their relationship isn't as simple as that. 


And I would say my favorite antagonist is 'Lee Jun Ho/Yeong' in 'I Remember You'. Man, that guy gave me the creeps, in both the adult and the teenage version. One of the best dramas out there as well. 
Before, for me, it was Mishil of Queen  Seon Deok because she was smart and strong and upfront. The type that fights and the winner can only be the best of the two, no coward, no stupid strike but really really wicked though. I didn't really "like" her but at the same time she commended respect. I love when a drama shows an enemy you respect, these are usually your greatest teachers. The drama went downhill without her. lol

Now my favorite is Gil Tae Mi of Six flying dragons. I love that he was not evil just for the sake of wickedness. He was fun as hell and actually very wise with his "clan". I never totally like antagonists but I do like it when they have cool aspects in their personality. I liked Gil Tae Mi more than Yi Bang Won actually ah ah ah !
Mishil and Kim Gil Do....they're such good villains. Mishil is so intelligent while Gil Do is a calming natural killer.