I was reminded of two more second leads I preferred because of others' posts, so I will add them.
1. Lin Yi Mu (Pretty Li Hui Zhen)
2. Choi Kang Hyeok (Flower Boy Ramen Shop): FL - "Should I, an adult woman, get involved with an entitled, arrogant, smartass, teenager who is a former student of mine - or should I choose the warm, sweet man closer to my own age who treats me well? Wow, what a conundrum..." I mean, I like Jung Il Woo, and I started watched the drama primarily because of him, but his character... not so much.
3. Baek In Ho (Cheese in the Trap)
4. Yeon Woo Young (My ID is Gangnam Beauty): Okay, I didn't actually dislike the male lead at all. He was pretty cool. But I liked Woo Young a lot more and found him more down-to-earth/authentic than the ML. He also handled the rejection so well. But the main couple is fine.
5. Kang Shin Woo (You're Beautiful)