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I am sorry to say that I am not that familiar about the Korean movies or artists who are acting in Korean movies. I have read an article from best essay writing service review blog which was written on Korean movie industry and then I came to know about how big is that industry and actors.

Nam Goong Min - Remember: War of Son
Jung Woong In - I Hear Your Voice
Shin Sung Rok - You Who Came from the Stars
Mishil - Queen Seon Deok!!! I loved her so much. Probably my favorite villain ever. 
Ki Cheol - Faith. This is a villain that I really appreciated the first time I watched but not as much the second time. idk I liked that he was vulnerable and we really got to see him struggle (not because of the main characters, but because of his own worries. It made him more like a character than just a villain whose only purpose is to make a plot.). I just appreciated that he had a purpose behind his motives I guess.
Mishil
any namgoongmin badass role
Yeo Woon (Yoo Seung Ho) in Warrior Baek Dong Soo :'(
Nobody thought Dong Ha from Suspicious Partner was really great? I was mesmerized.
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I second SWDBS The guy from Strong Woman was pretty creepy. Good acting, though. 
Lee Sung Jae from Gu Family Book  (tough to be for pure evil)
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Mishil (Go Hyunjung) from Queen Seon Deok
Chae Do Woo (Kim Kang Woo) from A Man's Story
YooJin (Song Yoona) from The K2
Taejoon (Yoo Jitae) from The Good Wife
Kim Bonggoo (Yoon Jemoon) from King 2 Heart
Shunji (park kiwoong) from Bridal Mask
Minjung (Lee Yuri) from jang bori is here
Dad/Killer (Kim Uinsung) from W: 2 worlds
Sung Hoon (Yang sejong) from Duel
Gwi (Lee Soo Hyuk) from Scholar Who Walks the Night
Taejoon (Jo JaeHyun) from Punch
Kwon Jae-hee (Nam Goong Min) from The Girl Who Sees Smells
jin pyo (Kim snagjoong) from city hunter
Seokhoon (yoon junghoon) from Mask
sooyoung (Joo soohyang) from school 2015: who are you
Park Bok Ja - Woman of Dignity
Jae Hyun - Manhole

they actually made me want to watch the drama because of them :D

Mo Tae Goo from Voice (Kim Jae Wook was really brilliant in this), one of my fav villains ever

 RoyalMajesty:

For Me:

Kwon Jae-hee (played by Nam Gung Min) from The Girl Who Sees Smells

Nam Gyu-man (played by Nam Gung Min) from Remember: War of the Son

Ditto, plus I'd add the character Cha Min Ho from The Defendant played by Uhm Ki Joon

Cho Gab Soo from Witch's Court

Spiritual Father from Save Me

the evil twin in Defendant (got the names mixed up xD)

Nam Gyu Man from Remember: War of the Son