Actor: Seo Kang Joon Whatever he is in, his ability to portray a part is next level up (and there's a huge amount of competition in South Korea because the line between character and actor is non existent).
Actress : IU Especially her last two dramas. Her subtlety at playing a part is unbelievable. Pure pleasure to watch.
Actor: Namgoong Min--- literally every drama I have seen him in is amazing and engrossing for the most part. He is a great actor and plays serious and dramatic roles very well. I am excited for his upcoming drama which has an interesting synopsis!
Actors: I feel like Yoo Ah In and Jung Kyung Ho generally chooses their projects well. The roles they play always provides new challenges to their filmography with roles that stand out from each other.
Also honorable mentions to Lee Sun Kyu, Namgoong Min and Song Kang Ho.
Actresses: Bae Doo Na and Lee Bo Young has taken part in some great projects, I've enjoyed a lot of the movies and series that they have chosen to act in. Two great actresses that picks shows with a lot of depth.
Also honorable mentions to Kim Tae Ri and Kim Go Eun who has been in some great stuff aswell.
I long ago gave up using "actor" and "actress" - after all, nobody says "doctress" any more, why insist on such an antiquated distinction? Having said that, the actor who stands out for me in consistently picking exceptionally well thought out roles that suit her style well is Lee Yo Won. Strong, independent, intelligent and driven characters are her forté, and she consistently nails them every time.