Here’s my list of my favorite saddest, or most impacting, Korean MV’s I’ve come across yet.
Some are in two parts. I’ll try to post the more violent/bloody ones towards the end for those who are squeamish.

First, here’s an overview list:

8eight – Goodbye My Lover
Kim Ji Eun – Yesterday is Different from Today
Gummy – I’m Sorry
Bom – You and I
Bom – Don’t Cry
Ailee – Heaven
Younha – Broke Up Today
Fly to the Sky – My Angel
Big Bang – Haru Haru
Davichi – Don’t Say Goodbye
Park Hyo Shin – After Love (Movie Version)
LOVEHOLIC – Sky
The Day Before – Nell
Position – By You Just Being In This World ft. Lee Wan
FT Island – A Man’s First Love & Only One (Part 2)
Bohemian – Don’t Hurt
2BiC – I Made Another Girl Cry
Bang Yong Gook  – I Remember
Zhang Li Yin – I Will (Part 1)
Zhang Li Yin – Lovers (Part 2)
Huh Gak – Hello (Part 1)
Huh Gak – I Told You I Wanna Die (Part 2)

Honorable Mentions:
Here are more that you should really check out. I just ran out of space to list all of them. Plus, there’s even more if you search for them!

Yangpa – It Hurts
2AM – You Wouldn’t Answer My Calls
Zhang Li Yin – Timeless (pt 1 & 2)
AB Avenue – Women Know
KISS – Because I’m A Girl
Jung Chul – It’s Raining
Taebin – The Reason I Close My Eyes
SAT – This Is Me
Sung Yuri – One Person
Ga-In – For You Not To Know
Brown Eyed Girls – Sign
8eight – Without a Heart
IU – The Story Only I Didn’t Know
Kim Dong Ryul – Replay
See Ya – A Woman’s Scent (there’s a part 2 prequel/sequel, but it’s not sad and it’s cliche)
Pray – SunnyHill

…And there’s so many videos featuring someone being hit by a car & then going back in time, that I’ll list them all here if you want to look them up:

Position – By You Just Being In This World ft. Lee Wan
JYJ – Heaven
Zia – The Way I Am
FT Island – Severely
Lee Hyun –  30 Minutes Ago ft.Lim Jeong Hee

Now get the tissues ready because here are the videos:

8eight – Goodbye My Lover
This is my favorite sad video of all time… it’s just as creative as it is tragic.

Kim Ji Eun – Yesterday is Different from Today
Tragedy… The worst part is knowing you have to let go of a love that’s so strong.
Someone also edited this video to “Don’t Go, Don’t Go” by Brown Eyes and that gives another dynamic.

Gummy – I’m Sorry

Bom – You and I

Bom – Don’t Cry

Ailee – Heaven

Younha – Broke Up Today

Fly to the Sky – My Angel
FYI The guy in this video has narcolepsy (chronic sleep disorder).

Big Bang – Haru Haru

Davichi – Don’t Say Goodbye

Park Hyo Shin – After Love

LOVEHOLIC – Sky

The Day Before – Nell
Even though this video is rather slow, the vocals along with the shock of the near end make it leave an impact on the viewer.

Position – By You Just Being In This World ft. Lee Wan
He cries… because he realizes what he has to do to save her.

FT Island – A Man’s First Love & Only One (Part 2)
Part One is just what would happen if he chose the other girl and it wasn’t quite as sad as this one.

Bohemian – Don’t Hurt

2BiC – I Made Another Girl Cry

Bang Yong Gook  – I Remember

__________________

Zhang Li Yin – I Will (Part 1)

Zhang Li Yin – Lovers (Part 2)

__________________

Huh Gak – Hello (Part 1)

Huh Gak – I Told You I Wanna Die (Part 2)