Castaway Diva> The childhood story was very good and the actors did really well.

Healer> The story of the parents and their friends was so sad. In a way, it could have been very interesting to watch a spin of with them but in the same time, their story was too tragic for me.

Start up> The set-up of the 2ML and the grandma was too well done and acted for a story remised in the second plan in the present.

Destined with You> The diffence of ton between the past and the present really made the past tragic and beautiful.

One comes to mind the most. Why? Because it was more powerful than the present day story being told.
I raise you Aeng-cho, played by Jo Bo-ah in Destines With You. Even though I was done with Hong-joo donkey episodes ago, the present-day character, the little time Aeng-cho spent on screen was not enough and never will be enough. To be honest, I preferred the backstory 1000× more than the present-day story it was telling. That's how gripping it was.

A close first is the backstory in See You In My 19th Life. Shin Hye-sun gave a raw visceral performance in the backstory that explained how the reincarnation began. It was quite a forceful backstory that was a story of its own.

I didn't watch this but I was a faithful recap reader: Moon In The Day.

If you want to talk way-back backstories, I loved how Chicago Typewriter used its reincarnation plot device to show the links between past and present resistance, and how art can be a critical tool for understanding and processing the long term effects of violence.

My favorite k-drama to explore how the past affects the present is probably Mixed Up Investigation Society (Evasive Inquiry Agency). It takes what seems like a silly premise (random cranks team up to find hidden gold) and then slowly peels back the layers to show how prior traumas, both historical and personal, continue to live on and shape our lives.

Hotel del Luna's sageuk backstory >>>>> present-day ghost of the week
The past storyline was the best part of the drama since the opening of Episode 1, and IU had the best romantic chemistry with Lee Do-hyun. The Hong sisters should've just written Hotel del Luna as a tragic sageuk romance.

In Love your Enemy the backstory was way more interesting than the present time story.

Other examples of good backstories:

_ Seasons of Blossom. Although I loved the two time lines.
_ What comes after love.
_ Go Back Couple.
_ Prison Playbook.
_ Flower of Evil.
_ Atypical Family.

Destined With You's backstory was more interesting and compelling than the present day plot, and it deserved more screen time than it got. Character/acting-wise, Aeng-cho was miles ahead of Hong-joo.

Love Your Enemy's backstory + the teen actors >>>>>>>>>>>>>>>> the present and the adult versions.

I loved the backstory in Like Flowers in Sand. The child actors really sold it too.

Also:
Call It Love
Way Back Love
Rain or Shine (Just Between Lovers)
Lovely Runner (aka Carry Sun Jae and Run)

Love Your Enemy
The flashbacks were far more interesting than the present timeline and the teen acctors were killing it.

Encounter - At the time I really resonated with the female lead’s backstory of feeling controlled and helpless which is why I really cheered when she finally finds her voice and starts choosing her own life. I still have a soft spot for this drama.

The backstory that is most memorable to me is of the character Eugene Choi in Mr. Sunshine. I have posted before of my love/hate feelings toward that show, but I will give credit where it's due. KES wrote incredible backstories for Eugene and many other characters in that series.

Connection

A few come to mind for me...

The Goryeo backstory in Goblin was SO good and interesting, that I struggled to muster up interest for anything in the present in that drama. It kind of made everything else meh...

The Legend of the Blue Sea was one of my earliest dramas, and I loved how the backstory was gradually revealed through dreams and flashbacks as the characters got entangled in the present. I think that ignited my love of the "intertwined past lives" trope.

I just started Just Between Lovers (which I'll watch slowwwwwly), and not only is the backstory compelling, but it's edited BEAUTIFULLY to function as memories, not just exposition. A really good example of narrative function meeting phenomenonal directing/editing work.

In Cheat On Me If You Can, I thought the backstory for the main leads was dripped out in a really enticing way. I wanted to know MORE the whole way through!

Then there are dramas that themselves kind of take place in the backstory, like My Perfect Stranger, Twinkling Watermelon, and Lovely Runner. I really enjoy those, too, especially when it's about the leads learning more about their parents.

Most recently probably ENA’s LIKE FLOWERS IN SAND (2023/24) (Netflix). The young actors were a delight.

Looking at my viewing log my No. 12 kdrama MBC’s PRIDE AND PREJUDICE (2014/15) (Kocowa) and my No. 13 KBS’s HELLO MONSTER/I REMEMBER YOU (Viki US) both revolved around (the trope) how the tragedies of youth play out in adulthood.
POSSIBLE SPOILERS FOR BOTH
P&P was particularly difficult because it revolved around a child’s kidnapping and murder.
HELLO MONSTER also was very difficult because it revolved around child mental illness.

Humor me please lol. On a happier HELLO MONSTER note some may recall on my fan wall I posted my all time favorite cast party pic. That was Park Bo-gum (adult Min) with his younger counterpart actor Hong Eun-tak (child Min):
https://41.media.tumblr.com/1e916d4a4852713837fd3977069ab363/tumblr_nsydji0VLZ1to1fhoo3_1280.jpg

I agree with a lot of these comments above--the backstory was more compelling than the main story in many of these dramas. But, for me, that is a problem endemic with kdramas, centered as the vast majority of them are on past life traumas as the sole explanation for characters' behavior.

In fact, for me, a lot of the time, its not that I get lost in the backstory, its that what I hoped would be an entertaining, feel good plot, instead gets tangled up in mawkish or maudlin past stories of some form or another--usually its some varient of parental abandonment or child-raising malpractice, which I kind of resent as someone who was a bumbling parent myself, but whose children turned out okay as adults regardless.

I'm not talking about "healing" dramas, like Unwritten Seoul. Of course, there, past traumas are so central to the plot that the point is to watch Park Bo Young suffer trauma twice, as twins. Rather, its romantic comedies that often bother me. What should be focused primarily on the chemistry of two people involved in a present day relationship instead gets hung up on the characters' past traumas. Last year, for example, with a few exceptions, almost every featured romance suffered, rather than benefited, from this narrative obsession with the damaging legacy of the past, beginning with Welcome to Samdal-ri and ending with Love Your Enemy. This year, it was My Dearest Nemesis and Tastefully Yours that got way too entangled in implausible responses to past events.

I love a good trauma as much as the next kdrama fan, but at times I have to roll my eyes. Especially since as I get older, the traumas that influence my behavior are not some vividly remembered incident from when I was an infant, but rather things like opening up a box of Melona bars and finding they had somehow melted before I got home. Now that's a tragic backstory that causes me to ugly cry!

The Glory -- Despite super painful subject matter, The Glory's protagonist's flashbacks to her trauma of bullying was riveting and glued me to my seat. Normally, the writers are trying to get rid of unpleasant subject matter asap (or touch lightly upon it or downright just tick a tickmark) to concentrate on the main story, but heroine's background was absolutely central to her development and present actions, and I was glad they spend sufficient time on that.

Beyond Evil -- I don't think there's a better psychological thriller that uses the background of the characters to blur lines between victim and criminal. Each one adds deeper layer to moral complexity that is so riverting and engaging. I was.seated.

I Can Hear Your Voice -- the shared traumatic background wasn't in itself quite as compelling as how it echoed later in each other's life. The flashbacks woven into the present-day story weren't there just to inform but they build up so much intensity, mixed with guilt and confusion, that I couldn't help but feel for these two.

Secret -- Ji Sung was superb as a wronged boyfriend who slowly falls for a woman who (supposedly) caused his gf demise. I am still squeeing at the painful background these two had to endure, and their happy ending was quite well deserved.

Squid Game -- I don't think this series would have worked if it hadn't given each player a compelling background. In fact, I craved finding out the background of each, their motivations, that compelled them to enter this high-stakes game. For me, this is the main mystery of the series, and why it worked so well for me.

This used to be a norm for kdramas, esp in sageuks. Remember those great times? So sad they're now gone(((

Biggest example must be MOON EMBRACES THE SUN, hands down. Adult part was both trainwreck and snooze-fest simultaneously, but teens? Teens were fantastic!

Personally I also really love TIME BETWEEN DOG AND WOLF's childhood intro episode, it just hit all the right notes. Didn't care much about adults though, they all sucked)))

Ok I was just about to comment Come and Hug me as a good example of a show using the teen life as a plot point but I guess that's not what we're discussing here, after reading the post. I think it helped that they gave the teen to adult story some room to breathe in CAHM as they were separated and later met again but then there were very few flashbacks once the story was set. I actually also like both past and present timelines in OUS , the past especially because young mi rae/mi ji does a great job and present because I'm invested in their story.

I enjoy dramas that are narratively interesting- dramas that are much more 'intertwined story' than 'backstory'
Healer has a story set in the past that is ensemble swashbuckling (pirate band fighting against the establishment) whereas the story in the present is jaded superhero. (Lone vigilante fighting on the edges of legality) The stories affect each other, but one is only 'backstory' because of the timeline and screen time. In narrative terms, I would weight them equally.
If 'backstory' is defined by where it is on a timeline, then Twinkling Watermelon is almost all backstory, with a small frontstory 'where are they now' intro and epilogue.
See you in my 19th Life has multiple backstories. If it had been constructed a bit better it could have been fantastic, but the writing/script editing is confusing. From a backstory point of view, better on a rewatch.
Call it Love and Just between Lovers both have what I would call a 'classic' backstory- A significant traumatic event in the past that shapes and affects interactions in the current day. They are both well written, but I don't feel the need to have more of either backstory.

Ok so just to clear, this prompt is not about identifying excellent backstories that elevated the present-day stories

This is about backstories that upstaged the present ones so much so the present-day stories fade into oblivion (or at least pale in comparison), yes?

Chicago Typewriter and Goblin are prime examples where the backstory clearly overshadowed the present time for me. In CT this manifested in a shoutout of 'yay!' whenever the scene went back to those revolutionaries and their struggles and in Goblin - At that time in the story, the tragic past of the characters combined with that intense performance .... well, I will forever wait for the combination of Kim Min Jae and Kim So-hyun. It still remains my most memorable part of the show.

Love Your Enemy has already been duly mentioned and is also a great example of a past way more interesting than the present.

It's not as fully fleshed out and there's no tragic backstory connecting the leads so maybe it doesn't count, but I like the dreamlike glimpses of the past in Because This is My First Life. Growing up female in a patriarchy, the economic conditions that shaped their generation, memories of the friendship between the three women. Somehow the fact that their relationship is the focus of the back story gives their friendship more weight, which I like. And I really love that it's all in the context of the beautiful poem she finds in a book in Se-Hee's closet, "To have a visitor / is indeed a matter of gravity. / For he / brings with him his past / present / plus / his future."

Hello everyone, the backstory of When my love blooms was more impactful that the present day story IMO.
That was the first time I watched Jeon So Nee and I've become a fan ever since. She had great chemistry with Park Jinyoung too.
The backstory was interesting because it dealt with the political turmoil of the 90s, student political involvement...

Thank you for all the great recommendations. Both Goblin and Come and Hug Me indeed have exceptional backstories.