There are so many great actors in Korean Drama. Among these actors, below are my choices for great actors for their acting prowess.

1. Jo In Sung 

- he really can act and knows how to convey the emotions of the character he portrays (Memories of Bali/That Winter, The Wind Blows/It's OK That's Love)

2. Joo Jin Mo

- very brilliant actor. I was swayed when played King in A Frozen Flower.

3. Park Bo Gum 

- After watching the first episode of Moonlight Drawn By Clouds, I became an instant fan of BoGummy. His strong presence playing the character as the Crown Prince is so refreshing to watch. I haven't seen an actor so young this talented.

4. Kim Woo Bin

- he knows how to throw emotions when a scene needs it.

5. Jo Hyun Jae - knows how to play with his eyes.

What about yours?

1. Seo In Guk 
- He is someone who can potray many acting styles. I was shocked watching him  in Shopping King Louie he acts silly, cute , funny. But If you watch I remember you he become a strong character. His eyes speaks alot , how he deliver his dialogues and emotion (mostly when he cried in the drama) . And in 38 Task Force , he showed his acting skills-genius, fraud . He lives up different characters ,not always the same.We can see how fine he acts as looking to his body language too . Every of his dramas never dissappoints me.
1. Song Kang Ho 
   No need for explanations, this ajusshi is the best Korean actor.

2. Jung Jae Young 
   Versatile actor, he can play any role. 
1. Jo Jung Suk
Have loved that man since a long time, and I'm glad he has JI right now

2. Choi Woo Shik 
Love his acting though his looks usually typecast him to fools roles

3. Seo In Guk 
Versatile, cute, funny, hot, anything you name it, he gives it.

4. Lee Won Geun 
Eye smile gets me everytime. hot young rising actor though he may have some awkward chemistry during romance scenes at times 
Wow, we are all so diverse with our picks. This is fun. :)
Lee Jun Ki I don't think I need to explain why ♥
Ji Sung
Im Joo Hwan
Lee Hyun Woo
Ji Soo
Ohn Joo Wan
Won Bin
Kim Bum
Jin Goo

(Following List is Best Acting+Great Cry Acting)
Jang Geun Suk
Joo Won
Ji Sung
Lee Jun Ki

I wasn't sure if you were using "actor" in the generic sense or the male sense so I've included my top 5 in each gender.

Lee Moon Shik / Park Se Young
Seo In Guk / Kim Hee Sun
Oh Jung Se / Kim Seul Gi
Jung Kyung Ho / Park Min Young
Min Sung Wook / Kim Mi Kyung
Lee Joon Gi / Ha Ji Won
Even though it's been a few years since I started kdramas, I don't really have actors I can say are my absolute faves. In terms of skills though, Cho Jin Woong really captivated me in Signal. One of the best performances I've seen personally!
Ji Soo - he plays tragic/badass characters very well and the small romantic scenes are very convincing. I hope he's a main in a romance drama some day
Lee Joon Gi - Moon Lovers, need I say more? I haven't watched enough of his other work but he was great in that
So Ji Sub - he has great chemistry with every romantic lead and is pretty funny
Kim Sae Ron - absolutely loved her in Mirror Of The Witch and Manhole. She's quite talented for her age
Lee Joon Gi - I think he knows what he's best at, sageuks. I first saw him in Arang and The Magistrate and has loved his works ever since. 

Seo In Guk - He caught my eye in the Reply series. I like that he doesn't go for 'similar' dramas, yet he excels in them all. 
1. Hwang Jung Min ( I bet a lot of non-Koreans don't know him but his acting is darn amazing)
2. Park Won Sang (He doesn't play main roles but he's a very talented supporting actor...He really can play any type of role. I really like this ahjussi's acting hahaha)
3. Kim Myung Min 
4. Byun Yohan
1. Kim Min Jong
2.Jang Hyuk
3.Ji Jin Hee
4.Bae Yong Joon
5.Oh Jung Se
6.Choi Jin Ho
7.Tae In Ho
Jung Kyung Ho
Ji Sung
Byun Yo Han
Park Bo Gum
Jo Jin Woong
Best actors and their best perfomance...

Ji Sung - Kill Me, Heal Me
Lee Joon Gi - Scarlet Heart Ryeo
Yoo Ah In - Six Flying Dragons
Ji Chang Wook - Empress Ki 
Joo Won - Good Doctor