I haven't seen many kdrama yet, but I'll list charas from the ones I have; gonna try to avoid mentioning anything spoilerish for my reasons
Yoon Dong Ha in Witch's Romance - it's surreal how perfect his character is, really. Like the perfect boyfriend.
Kim Shin Hyeok in She Was Pretty - Although I ultimately did sail on the Seong Joon and Hye Jin ship, I had major second lead syndrome for the majority of this drama. Siwon's character was just so funny and charismatic.
Oh Ri On in Kill Me, Heal Me - "COME ON SHISTERRRRR"
Seol Hee in Fight For My Way - I didn't really relate to her much throughout the series as much as other characters, but I sympathized so much for her, and glad her character progressed the way she did.