For me, the most unforgettable performance was Kim soo hyun in its okay not to be okay. I always knew that Kim soo Hyun was a good actor. Seo ye ji and Oh jung se were great too. However, the scene where Sang tae accuses Kang tae of trying to kill him when they were children......... That's when I realized that Kim soo hyun is an ACTOR (all caps required).

Another unforgettable performance was IU in My mister. She was mind-blowingly good especially because I didn't know she had it in her. (I had only seen her in Scarlet Heart before that)

And now Shin Hye sun in See you in my 19th life, especially the scene in the screencap......

I just feel lucky to be witnessing such greatness.

The death scene by the shoreline, The Crowned Clown, episode 8. Yeo Jin- goo. So tragic, he is killed by poison by the person he trusts most, even at the very end.

I’ll never forget Oh Jung-se’s performance in Hot Stove League, on the scene wherein his pent-up angst (caused by his cousin’s looking down on him and his father) got all released during their arm wrestle. The facial acting, the tears… just wow! I know he is a great actor, but this performance made me think that there’s no way he had/would ever deliver/ed a bad performance.

Supporting actor who steals every scene: It will always be Lee Kyu-hyung for me as Looney in Prison Playbook. He made the role so charming despite being a junkie.

Memorable performance 1: Park Eun-bin as attorney Woo in Extraordinary Attorney Woo.

2: Child actor Kim Kang-hoon and Lee Sung-min in Reborn Rich. Lee Sung-min was totally charismatic in his role, and no surprises there since he's that good of an actor. But I was pleasantly surprised to see Kang-hoon hold his own against the veteran actor, and it was amazing how they both bounced off each other in their scenes together. Really impressive stuff!

3. Kim Soo-hyun in everything! ❤️

Jang Hyuk as Dae Gil in Chuno. Heart wrenching and unforgettable.

Park Ki Woong in Gaksital
Yeo Jin Too in The Crowned Clown (adult) and Giant (child)
Yang Se Jong in Duel

Shin Hye Sun in 30 but 17 and Mr. Queen
Kim Sun A in Red Moon Blue Sun

I can't pick only one, but fortunately this is not a "you can pick only one discussion", so... let me expand on this comment a bit XD

Kim Nam Gil in Queen Seon Deok.
Yoon Si Yoon in everything (Nokdu Flower and You Raise Me Up have some moments I know I won't forget).
Hyun Bin in Memories of Alhambra.
Kang Hoon in Secret Romantic Guesthouse.
Byun Yo Han in Misaeng and Six Flying Dragons.
Choi Hyun Wook and Park Ji Hoon in Weak Hero Class 1.
Lee Kyu Hyung in Prison Playbook.
Kim Tae Ri in Twenty-five Twenty-one.
Jang Hyuk in Fated to love you.
Lee Seung Gi un Hwayugi.
Lee Joon Gi in Flower of Evil.
Park Eun Bin in Age of Youth 1 and 2.
Seo In Guk in Shopping King Louis.

In terms of a whole drama, I’m going to go with Park Ho-san as Park Seok in Would You Like a Cup of Coffee? I feel this is cheating a bit, because that drama is so delightful, soft, and thoughtful throughout, but Park Ho-san’s performance as the steadfast, strong, and watchful coffee shop owner was outstanding. He was the bedrock for his close community--a found family, really--as well as for that show to me.

In terms of an individual scene, the performance that lives completely fixed in memory is the scene in Thirty-Nine where Jin-seok (Lee Moo-saeng) begs through his grief to be allowed to live and stay by the side of Chan-young (Jeon Mi-do) as the tragedies of their past and their present comes crashing down around them both.

This drama has undergone many, many negative critiques, some of which I agree with, some of which I do not...but when I read this post's prompt, this particular performance was a no-brainer for me. These few minutes not only had me burst into ugly tears myself, I even had to step away from the drama for some time to actually process all of what I’d just experienced in my own mind and body. Truly impactful on me.

To me, Shin Hye-Sun is talented and skilled as an actress extraordinaire, given her young age.

As pointed out by @snow flower, no other actress can do what she did in 30 but 17 and Mr Queen. These are two very unique characters that are hard to bring alive but she delivers to the T.

Then just to make sure this is on the list - Cho Seung Woo. He creates a different persona and mannerism for each role he plays. Think Hwang Shi-mo in Forest of Secrets x 2, Gu Seung-hyo in Life, and Divorced Attoeney Shin. Can’t they be more different?!

So many good ones already mentioned, so I'll add:
It's OK That's Love: Zo In-sung
Frozen Flower:Zo In-sung, Joo Jin-mo, Song Ji-hyo
Que Sera Sera: Eric Mun
Return of Iljimae: Jung Il Woo

Dramaland is rife with standout performances, which makes it hard to narrow it down for a list! Like Missvictrix, I'll second Shin Hye Sun - she always gives 100%. I was very impressed with the 19th Life scene referenced here. Recently, I was also super impressed with Woo Do Hwan in Joseon Attorney. The drama itself was uneven, but he was phenomenal, and his acting in ep 14 was heartbreakingly good. Not a recent drama, but a recent watch for me - Yoon Shi Yoon in Psychopath Diary. It was a challenging role, and he nailed the tonal shifts and emotional complexity. For other dramas, I think of ML/FL pairings where they elevate each other to new heights of excellence... Kim Young Kwang and Lee Sung Kyung in Call it Love; Jung Il Woo and Kwon Yuri in Bossam: Steal the Fate (I always remember the prison scene where they're on either side of the bars and are crying while just Looking at each other). And there are many others!

Lee Sung-min in Reborn Rich

So many actors... so many names that I will have to search to write them correctly...

For their acting in the whole drama :
Cho Seung-woo in Forest of Secrets 1 and 2
Kim Nam-gil in Through the Darkness
Choi Min-Soo in Extracurricular

For some scenes in their dramas :
Kim Yoo-Jung in Backstreet Rookie or May Queen or any scene she's crying. Everytime she makes me cry too.
Yeo Jin-Goo in Crowned Clown, the beach scene was incredible thanks to the actors and the PD.
Jung Hae-In in D.P. the scene of the tunnel or the one when he hits the corporal
Park Eun-Bin and Rowoon in The King's Affection's pond scene

I saw a couple of KIM NAM GIL as Bidams here, but my fave performance of his was the interview scene in in THROUGH THE DARKNESS ep8 when he interviews Young Chun. That was the pivotal moment when Ha Young cracked the code of his job. The tonal shift from an unsure detective to a confident profiler was so subtle yet palpable, something unachievable in the hands of a weaker actor. Another fave was not in a drama, but a movie -- NO REGRET. The portrayal of love was so raw and natural, I didn't see him acting at all, just being.

There are many great actors and actresses in Kdramaland, but for some characters to be elevated to masterpieces, I think of:

1. Cho Seung-woo as Hwang Shi-mok in Forest of Secrets 1.
2. Kim Nam-gil as Heo Im in Live Up to Your Name
3. Han Suk-kyu as Doctor Rimantic Kim in Romantic Doctor Teacher Kim
4. Kang Ha-neul in When the Camellia Blooms
5. Nam Goong-min as the antagonist in Remember - Son’s War
6. Seo Hyun-jin in Another Oh Hae-young
7. Kim Tae-ri and Bona in 25 21

I think I’ll stop here or the list would go on forever. Coming to think about it, there are A LOT of great and memorable characters in Kdramaland.

Yeo Jin-goo in Circle - throughout that show, but the scene that stands out for me is when he's begging his brother to give up on his pursuit of aliens and return to reality and a normal life. It's hard to believe he was only 19 when he filmed that. He's also fantastic in Beyond Evil, as is Shin Ha-kyun; they really brought out the best in each other.

Jo Seung-woo rightfully gets praised for his role in Stranger, and he was extraordinary, but Bae Doona was amazing too.

Gong Yoo -Goblin, episode 16 when he meets Eun Tak in Reaper’s Tea Room. His eyes, facial expressions, body posture and sounds/noises of grief are so impactful, it’s heartbreaking to watch. Every time I watch the scene I end up a blubbering mess as it feels so real.

Lee Dong Wook- Goblin, episode 12 and 13 in period costume as the downward spiralling manic, obsessive king, Wang Yeo. Just… wow, so believable and absolutely fantastic.
The best I have ever seen him act to this day.
In Episode 12, Lee Dong Wook as Wang Yeo asking his queen Yoo In-ah to choose between himself or her brother, the traitor. She chooses her brother. The acting showing the complex emotions is simply mesmerising in its intensity. The best I have seen of Yoo In-ah as well.

Several actors/actresses whose performances are always "brilliant" in any role:
Kang Ha-Neul (Miseang, When the Camellia Blooms)
Yeo Jin-Goo (Crowned Clown)
Kim Sung-Cheol (Prison Playbook) talented musician, too!
Lee Je-Hoon (Signal, Move to Heaven)
-Kim Hye-Sun (Mr Queen)
-Seo Hyun-Jin (Another Miss Oh)
-Park Eun-Bin (Extraordinary Attorney Woo)
-IU (My Mister)