Here's a kinda sorta top 10 :)
Ko Chia Yen in Someday or One Day
Shibasaki Kou in Orange Days
Ma Si Chun in Mr Fighting and Soul Mate
Cho Seung Woo in Forest of Secrets and Forest of Secrets 2
Kiyohara Kaya in Okaeri Mone
Jian Man Shu in The Arc of Life
Zhou Dong Yu in Soul Mate
Lee Bo Young in Agency
Jo Bo Ah in Goodbye to Goodbye
Koshiba Fuka in Yuganda Hamon
In no particular order and too many to choose from but these are the most memorable to me so far for their preparation for the role and the emotions that they make me feel (good or bad)
But really, I just get impressed when I watch several of one actor's works back to back and the characters in each are very distinct. At the moment, thanks to Goblin, Gong Yoo and Lee Dong Wook are on my radar. The former through a comparison between his role as Goblin and the MC in Train to Busan, more so given they released in the same year. The latter between his role as Reaper and the secretive uncle in A Shop For Killers which I just finished binging this morning.
I agree with Kim Soo-hyun, he's a great actor. Some other performances that I was blown away by are:
I'm not sure if I'd have the same opinion now since it's been a while since I watched it but Ahn Bohyun in Itaewon Class was phenomenal! His character was genuinely insane to the point where watching his scenes I was filled with a feeling of shock and disgust. I've never hated a character as much as I hated his.
I also really liked Shin Hyesun in Welcome to Samdalri. It wasn't the best drama but she really shined.... the whole time I kept thinking "wow, this is real acting". I definitely have to see more of her dramas!
Of course Ji Changwook has to be on this list too! His acting is so captivating to me, especially in Lovestruck in the City and the last few episodes of The Sound of Magic. I like that he can switch between so many different characters but you can still feel a piece of him in everyone he plays. Definitely a true performer.
Kim soo hyun in Queen of tears. Cried with him in every scene!!! HE just nailed it.
Throwing love to some less appreciated actors here:
Moon Chae Won in The Princess Man, Innocent Man, Flower of Evil - no one has the perfect voice and acting to show how desparately she loves her man, no one. She is one natural underappreciated actress. It was a travesty to not nominate her for Flower of Evil (and I really hate it that she's not getting the same love as the male lead for the drama), but unlike some people who think the same of her non-nomination, Kim So Hyun of River Where the Moon Rises is not the actress for me who should be removed from the list of actual nominees.
Moon Geun Young also in Painter of the Wind - no one for me is as good as her in acting like a man, she always conveyed all the expressions her character needs to express perfectly, no more and no less. She and Moon Chae Won have electric chemistry but she and Park Shin Yang also have a fun and friendly chemistry that you'll also find yourself somehow rooting for them even if you find their age gap weird at first.