I’m looking for a kdrama that is good in all other areas but shines because of the natural chemistry between the leads. I don’t really care what genre it is other than that but I do like happy endings so if you recommend something that is sad, please don’t let it have a sad ending.
P.S. I really loved Ji Chang Wook in Healer and K2 for this reason. He and the female leads had that kind of easy going chemistry.
Secret Garden 
Witch’s Romance
Suspicious Partner
It’s Okay it’s Love
All In
Flower Boy Ramen Shop
Jealousy Incarnate
Coffee Prince
Fated to Love You
I know you said Korean dramas but these are good too
Chinese dramas
Love Me If You Dare
When a Snail Falls In Love
Taiwanese dramas
Murphy’s Law of Romance (4 minute kiss)
Fall in Love with Me ( great kissing)
= chemistry meter out of 5
Suspicious Partner
Fight My Way
Another Oh Hae Young
Marriage, Not Dating - totally believable leads-I didn’t want their story to end
Oh My Ghostess - great chemistry here
Strong Woman Do Bong Soon - so cute that even the crew thought they were dating in real life during filming
Secret - Ji Sung & Hwang Jung Em nailed it here
Weightlifting Fairy Kim Bok Joo - main couple now dating in real life and we see why in this drama
If you’re willing to consider TW drama"
Just You - Aron Yan & Puff Gao good friends in real life, and it shows here
If you willing to look at Thai remake of Korean drama-
Full House Thai - fantastic chemistry between the 2 leads, they are also good friends in real life