So Ji Sub and Jo In Sung..
Those two make my heart melt like chocolate... 
Rather than a favorite actor, I find that I love an actor in a particular
character -  a combination of great acting and great character.  
Chun Jung Myung - Heart to Heart
Song Joong Ki - Innocent Man
Siwan - Misaeng
Lee Jin Wook - 9 Times Time Travel, The Threee Musketeers
Jung Woo Sung - Padam, Padam
Lee Min Ho - Personal Taste
Hyun Bin - Secret Garden
Lee Jun Ki - Time Between Dog and Wolf, Two Weeks, Scholar who Walks the night
Yoon Kye Sang - Beyond the Clouds, Who are You?
Gong Yoo - Coffee Prince
Jung Kyung Ho - Cruel City
Ji Jin Hee - Dae Jang Geum
Yoon Sang Hyun - Gap Dong
Choi Jin Hyuk - Gu Family Book
Ji Chang Wook - Healer
Lee Dong Wook - Hotel King
So Ji Sub - Road No. 1, The Sun of My Master
Yoo Ah In - Secret Love Affair
Jo In Sung - That Winter the Wind Blows, It's Ok it's Love
Kim Soo Hyun - The Moon that Embraces the Sun, My Love who came from the stars
Jung Il Woo - The Return of IIjimae
Kim Jae Joong - Triangle
Im Ju Hwan - Ugly Alert
Yoo Seung Ho - Warrior Baek Dong Soo, God of Study
Cha Seung Won - The Greatest Lover, You are all Surrounded
Shin Da Eun is my favourite. The others I like too.
Kim Sun-ah
Jung Jin-young
Oh Yoon-ah
I may post others,
So Ji Sub
Jang Hyuk
Lee Jun Ki
Seo In Guk
Park Bo Gum He is just soo cute (especially in Reply 1988) 

Seo In Guk  He has the talent that caught my attention. His looks and voice also is attracting

Kim Soo Hyun He has the skills! I mean...he makes a lot of mistakes behind the scenes but I just like him when he acts! ^_^

And there are a lot more, but these are my TOP 3!!!
also jisung, lee jongsuk, ji chang wook, jang geun suk etc
Jung Kyung Ho (Cruel City, Falling for Innocence)
Ji Sung (Kill Me, Heal Me, Secret)
Kim Bum (Boys over flowers)
So Ji Sub (Ghost)
Jo Seung Woo (God's Gift - 14 Days)
So Ji Sub
Seo In Guk
Joo Won
Kim Soo Hyun
Lee Jong Suk
Ji Chang Wook
Seo Kang Joon
Ji Sung
Yoon Shi Yoon
Kim Woo Bin
Sung Joon
Lee Jong Suk
Hyun Bin

And probably SO many more haha.
Yoo Seung Ho - The warrior Beak Dong Soo, Imaginary Cat & Remember 
Kim Hyun Joong - Playful kiss 
Lee Won Gun - Sassy Go Go
Ji Chang Wook - Healer 
Lee Jong Suk - Doctor stranger, school 2013 ans Pinocchio 
Park Bo Gum & Seo In Guk - I remember you 
Le Hyun Woo - You're beautiful & Moorim School 
Jung Il Woo - The night watchmen & 49 days 
Lee Min Ho - City Hunter
Lee Seun Gi - Gu Book family, My girlfriend is Gumiho etc..

Ooohhh, don't know too many, but Park Yoochun, Namgung Min, Seo In Guk, So Ji Sub, and Park Bo Gum!
gong yoo ...jo in sung ..hyun bin ..so ji sub...ji chan wook ....kim hyun joong  i know he's not a full actor but i love him ^^....jang hyuk ....there's a lot but those who i can remember at this moment ^^
Won Bin            - Autumn Tale, The Man From Nowhere, Tae Guk Gi: The Brotherhood of War, 
Jung Woo Sung - Daisy ,Sad movie , cold eyes
Song Kang Ho   - The attorney, Howling
So Ji Sub          - Always
Song Joong Ki   - A Werewolf Boy, Sungkyunkwan Scandal
Jang Geun Suk  - Hwang Jin Y, Mary Stayed Out All Night, Do Re Mi Fa So La Ti Do, Love Rain
Lee Joon Gi       - May 18
Yoo Seung Ho   - Blind
Lee Min Ho        - Personal Taste, City Hunter
Song Seung Hun-He Was Cool
Jo In Sung         -Classic

quite a lot ;)
Joo Won, Lee Jong Suk, and Lee Jung Ki