Ha Ji Won and Yoon Eun Hye
I have many favorite korean actresses so i'll just put it on my profile. :) It can be found der.

Top 2
Moon Chae Won- Love her in Good Doctor and Innocent man
Hwang Jung Eum- Amazing in Can you Hear My Heart, Secret, and Kill Me Heal Me

Then
Jang Nara- Excellent in Fated to Love You
Go Ara- As a rookie she did excellent in Reply 1994
Jung Eun Ji- great acting in Reply 1997
Gong Hyo Jin- Love her natural acting (Master's Sun, Producers, It's Okay That's Love)

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Lee Jong Suk Nam Joo Hyuk Kim Woo Bin Sung Joon Lee Min Ho
Park Shin Hye
Han Hyo Joo
Gong Hyo Jin
and Hwang Jang Eum KILLED it in Kill Me Heal Me
jun ji hyun,ha ji won, moon chae won
hwang jung eum - because of kmhm
han hyo joo

Yoon Eun Hye
Jeon Ji Hyeon
Kim Sun Ah
Gong Hyo Jin
Yoo In Na
Ha Ji Won
Moon Geun Young
So far I've pretty much only loved actresses for one role only. 
However those I really adore and that I find absolutely beautiful are Shin Min Ah and Park Bo Young.

Then I loved Jeon Ji Hyeon in YWCFS, Park Min Young in CH, Jung Eun Ji in Reply 1997, and Jung Yu Mi in QSS!
MY TOP 10

Jang Na Ra
Jeon Ji Hyun
Jung Eun Ji
Lee Bo Young
Ha Ji Won
Park Bo Young
Kim Mi Kyung
Go Ah Ra
Gong Hyo Jin
Son Ye Jin
My favs are Jeon Ji Hyeon, Park Min Young and Jang Na Ra
Hmmm hard to decide between Park Min Young and Park Shin Hye
Lee Da Hae
Choi Ji Won
Park Min Young
Im Soo Hyang
Soo Ae
Lee Bo Young
Jeon Ji Hyeon
Go Joon Hee.