"Heart of Ice" is often regarded as the greatest episode of Batman: The Animated Series. In this beautifully-directed story, Mr. Freeze is the most sympathetic of Batman villains.
It was a wrenching episode ranked among Watch Mojo's grand list of the 10 saddest cartoon episodes ever. Among them are tragedies from The Simpsons, Family Guy, Rugrats, and, most importantly, the Ice King origin story from Adventure Time.
In a longer list, I would have included the Exosquad episode "Ultimatum." James Burns, a leader of the Venusian resistence, dies gallantly fighting his own people. James is finally at rest. But now his sister, Nara Burns, has lost her entire family in the war. Later, things get worse for her.
-via Geek Tyrant
I could watch it only once.
"When the wind blows"
and
"Grave of the Fireflies"