Grave of the Fireflies is, by far, the saddest movie I have ever watched. It makes Titanic look like a romantic comedy. Simba's death in The Lion King is a joke compared to Grave of the Fireflies. The wife's death in Up pales in comparison to the tragedies that befall the main characters in Grave of the Fireflies.
From the very start, the main characters face tragedy after tragedy, hardship after hardship. When you think they might finally be escaping from the terrible cycle they're in, it just gets worse. And worse. And worse. Then the unthinkable happens. Then the unthinkable happens again. And again after that. And just when you think "this is it, this is going to be their breaking point and nothing more bad will happen to them", it gets worse.
The movie perfectly captures the suffering and hardships of what would happen to two siblings caught in the final months of WWII after losing their parents. It's honestly very difficult to watch at times, and hits just as hard as real footage of real humans suffering in terrible living conditions or poverty. This absolute visceral feeling it invokes is just why it is a nearly perfect animation. I have yet to watch another movie that kept me crying nonstop for as long as this one did. If you want to watch something sad, on another level from typical sad movies, watch Grave of the Fireflies. I guarantee you will cry, and if you don't cry, you will punch a hole in your wall.
Beware, though. There is no good ending in this movie. It does not end with the characters finding their way out of a terrible situation and moving on with their lives. They suffer from start to finish, and you will not get a feeling of "good ending" satisfaction from this masterpiece. It will leave you with a bitter taste in your mouth. I would not recommend watching this movie if you have anything else to do that day, either, because you're not going to want to go do it anymore.