The small beating the big, the few winning against the many are legends and something which do not occur easily.
1.
The greatest upset of Korean drama matchups is "Purity" beating "White Night 3.98" in 1998.
Since it occurred too long ago, most people won't be familiar with this story.
Long story short, KBS' Purity, a melodrama with few casts and little expenses starring Ryu Shiwon (then a star of melodramas) and Myung Sebin, defeated White Night 3.98, a drama supposedly involving a North-South Korean spy rivalry encompassing the Koreas, China , Central Asia and Russia , trying to steal a super fast warplan with a speed of Mach 3.98. It starred Lee Byunghun , Shim Eunha, Choi Minsoo and numerous others (including Song Hyekyo in a very minor role).
It was a fishing trawler against an aircraft carrier, with the latter outspending the former about 10:1, but long story short, Purity sank White Night 3.98. Melodrama worked during the year 1998 when a lot of Koreans were suffering from a major economic crisis.
2.
The Greatest Upset of drama matchups in the 21st century is the matchup between Love in Moonlight (KBS) and Moon Lovers : Scarlet Heart Ryeo (SBS).
Ryeo featured Lee Joongi, Kang Haneul, Lee Jieun, Hong Jonghyun, Nam Joohyuk, Byun Baekhyun(EXO) and many others. It was invested by a Chinese company and also YG. with a well known Chinese original.
Moonlight featured Park Bogum, Kim Yoojung ... and others.
Ryeo spent at least $15 million . That's 'at least'.
Moonlight, produced by KBS itself, spent around $5 million.
a 3-1 difference.
But the battle was decided in the first episode, focusing on IU and Baekhyun, which was seen as a sitcom by the viewers who turned to Moonlight.
Park Bogum became Korea's biggest star.
Ryeo did a bit batter in the end, but it was too little and too late. SBS has never attempted any major drama projects since then because the failure of Ryeo was so hurting.
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3.
Another great upset occurred on 2019. Curiously, it was not a matchup between dramas which were aired at the same time.
Arthdal Chronicles was Korea's first real attempt on fantasy world dramas, starring Song Joongi, Jang Donggun, Kim Jiwon and others.
It spent at least $50 million, unheard in Korean production history.
and, lo and behold, a lot of people raised questions that it was too like the Game of Thrones adapted to Korea.
It showed lots and lots of kinks, and it became so big that after airing the second installment, its staff had to stop and wait till the third part was completed.
To fill the gap , Hotel Del Luna was entered, starting .... the aforementioned Lee Jieun, who had gained a Grand Prize in singing but despite of My Mister, was not seen to be someone who could lead a drama.
Hotel Del Luna was basically made with the leftovers from Arthdal . With no major stars other than IU, she basically led the drama from the beginning to the end.
After HDL ended, the final installment of the "FIRST PART" of Arthdal aired, with no major stir.
Hotel Del Luna was TVN's highest rated program in 2019.
Arthdal spent $50 million, while HDL , mostly using Arthdal's equipments and staff, cost maybe 1/5 of it.
Probably the greatest upset in tvn's history ever.