The Reveal that Ying Ying has died. Her funeral, too.
The scene in which Ling's spirit is finally put to rest.
Not to mention the aftermath of the enormous gas tanker explosion. The sight of all those poor, burnt corpses is incredibly haunting and heartbreaking.
Mun's scene with Ying Ying, shortly after she's just has chemotherapy, is utterly heart-rending, especially on a repeat viewing, when one takes into account that it's the last time they ever speak - when Ying Ying is still alive, anyway. They never get to play together outside of the hospital, after all...