Madam Choi, SONG YOON AH in K2 MY GOD She is wonderful in crying. I feel every sense of betrayal, anger, sadness in her tears it's crazy. I'm such a fan of her despite her age. It's not beautiful aesthetically but beautiful in a sense where she moved my heart which transcends way beyond crying to look pretty. 
Ji Sung 
Jo Jin Woong 
Kim Go Eun's crying is heartbreaking. It's so real and raw. Park Shin Hye's crying always gets me too.
Moon chae won 
Jun ji hyun 
Gong yoo
Jo in sung
Kim go eun
yoo seung ho
hwang jung eum
kim soo hyun
yoon eun hye
yeo jin goo
Kang Sora in dream high haha

Jo Byeong Gyu - Uncanny encounter

Nam Goong Min - Falsify

Esom - In taxi driver

Junho (2PM) - In Confession

Go Joon - Fiery Priest

 AreumYoung:
Park Bo Young
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Moon Chae Won
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The best of all, for me hehe~

I'm impressed with almost everyone's cry except Park Bo Young.  She's so over-the-top that I vacillate between irritation and laughter.

Ahn Bo Hyun and Jo bo Ah for me.

Especially if you've seen that scene with Jo Bo Ah in Missing Noir M. She's stuck me as an actress who knows how to cry. 

Shin Hye Sun is a phenomenal actress, whose ability to invoke raw emotion on screen rivals some of the best, but for me she is one of the best to ever do it.

Gong Yoo and Kim Go Eun in Goblin.
Their solo sobs were unmatched by anything I've seen before and after.

https://www.youtube.com/watch?v=4-FQ5ySoY9M
https://www.youtube.com/watch?v=5JteB30DAN4