Hotel Del Luna- Chung Myung  (He deserved Man Weol  after staying by her side for 1,300 years.)

Cheese in the Trap- In Ho

While You Were Sleeping- Woo Tak

My First First Love 2-  Do Hyun

Meteor Garden 2018- Lei 

Love Me If You Dare- Xun Ran

Koi wa Tsuzuku yo Doko Made mo- Koichi

The King: Eternal Monarch- Sinjae

I was sad when Chun-soo from Emergency Couple didn't get together with Jin-hee.

I think there are a lot of Second ML deserving the FL but the number one would be Jung-hwan from Reply 1988 LOL

Who are You: School 2015 - I was so sad when the FL didn't end up with Gong Tae Kwang (the second ML). The chemistry between the two was just amazing.....still not over it even though it been years since I watched it XD. 

Kim Shin Hyuk from She was pretty

The second ML from Itazura na kiss and its various adaptations.

What immediately comes to mind is Baek In Ho from "Cheese in the Trap" and Nak Goo from "I Picked Up a Star on the Road".

Jealousy Incarnate - Go Jeong Won (Go Kyung Pyo)

You're Beautiful - Kang Shin Woo (Jung Yong Hwa)

Meteor Garden 2018 - Hua Ze Lei (Darren Chen)

Reply 1994 Chilbong deserves Najung better!! 

Hana Yori Dango and all its versions - Hanazawa Rui totally deserved the FL rather than the ML. His sad story didn't give him any rights to torture others

Boys over Flowers - Ji Hoo and Jandi

How about,
Mr Fox and Miss Rose- Yi tian?

My ambulance : here the main lead deserved the FL but SML got her

Itazura na Kiss! Kin-chan deserved Kotoko much more than Irie-kun 

Who Are You: School 2015

Gong Tae Kwang deserve better, he was friendzoned by Eun Byul. I'm still not over it until now, LOL. That's why I want a korean drama of Kim So Hyun and Yook Sung Jae and by then.. they should end up together.