They all are a good and the best actors. The choices not really hard for me, mostly they are not my bias. Just 2 of them that get my eyes. are So Ji Sub and Lee Dong Wook. 
But if I should choose one, I'll be point on

SO JI SUB

not hard at all. hihihihihi
the hardest ever, i really enjoy watching their dramas...
but for today i choose......

Park Seo Joon ^^
Well, my favorite is.. His first name is Gongyoojanghyuksojisubjisungparkseojoon and his last name is Leeminhoparkhaejinleedongwookleejungijungkyungho.
It wasn't a difficult choice. Why are you guys struggling? XD ♡
right now it's Park Hae Jin 
Jung Kyung Ho And Park hae jin ! 
I learned to count like @Sunah (13th comment)    lol  x)