I will say though as much as I love love triangles. I also like shows without them. Healer was a great story and had its own conflicts and I'm glad there was no second lead in it to form a triangle. Also, no Second Lead makes it easier on my heart!
I think who I am as a person I always root for the 2nd lead. Like ALWAYS. In oh my ghostess and omggg it was so strong in cheese in the Trap. And even though I couldn't finish Jealousy incarnate SLS was super strong
Oh my, the 'She was pretty' second lead syndrome hit me hard. But honestly I didn't want him with the main character, I just rooted for his happiness and that the girl would stop making him confused and hurt. GIVE SHI WON THE LOVE HE DESERVES. Imo she had a really bad attitude towards him.
To the Beautiful You - I'm still not able to touch this drama, because every time I see Lee Hyun Woo's face on the screen my heart melts and cries. My biggest SLS ever.
As everyone's already mentioned, School 2015...actually, the "end-game couple" (who's probably not actually end-game outside of the show, I mean, they were in high school) was really silly. I know the writers probably didn't want to do the cliche of letting the two guys be with the two girls but that doesn't mean they needed to make someone in love with a different person all his life switch teams at the very end, instead of the one who recognized her and loved her for who she truly was. Another major one was Sungkyunkwan Scandal. It's not even my bias against the actor who plays the male lead (even though his projects are pretty much ruined for me) it's just that Yoon Hee and Geol Oh had so much more chemistry & he was just the better, more moving character all around. I think the kind of SLS that most writers strive for is the one you get while watching the child versions of The Moon That Embraces The Sun: you love the entire triangle and want them all to be happy and it breaks your heart that they can't all be & you do prefer the main pairing but ;______; don't want it to be at the expense of the other dude, a mark that's often missed by viewers just preferring the second lead completely.
Also, tbh I've never seen someone have SLS from R97. The others, sure, it's a closer battle there, but not really R97? Seriously, has anyone?