1. Lin Yi Mu (Pretty Li Hui Zhen): I sort of shipped the main character and her best friend, but it's a Chinese drama, so more realistically, I would have much preferred seeing her end up with this character. They had a much more relaxed, playful, cute dynamic overall. The ML was so terribly written in this drama that I alternated between being frustrated and bored by the canon relationship.
2. Baek In Ho (Cheese in the Trap): As I recall, the ML was kind of a sociopath. The second lead was a talented musician and relatively normal, relatable character.
3. Kang Shin Woo (You're Beautiful): The ML wasn't completely terrible, but he was often arrogant and rude. Shin Woo was kind, gentle, and trustworthy. I preferred him overall, but I didn't like that he kept trying to push the FL to return his feelings when she made it clear she wasn't interested.
That's all I've got, and I had to include a Chinese drama to even get three. I actually don't experience second lead syndrome too often. It usually only happens when I find the ML particularly unappealing and/or poorly written.