I really really like any SML actor, they have the worst part in the drama. My heart is broken today after watching "Captain of Destiny" and I've almost cried over him. 

This is an old post, but just coming across it, 2ML (Won Bin) in https://mydramalist.com/255-autumn-tale  wins as the best 2ML.   I saw this when I was little with my mom, and still remember thinking that the 2ML out shining the main lead.  He was WAY better than the ML. 

More recently, I also liked the smart 2ML in Love Like the Galaxy.  He wasnt better than the ML,  and I'm glad that the ML got the girl, but I felt terrible for him.