I actually searched this thread before I posted wondering if somebody would mention this. It's definitely way sadder of a film than most of the ones posted here, IMO.
the saddest movie that i've watched is grave of the fireflies. my sophomore year high school english teacher was showing it to my class and i heard it was a depressing movie so i went home, watched it on my laptop, cried (a lot), went back to school, and tried to watch it in class without crying again
Saddest moments:
Bambi's mum dies
Mufasa dies
When Todd if left in the woods in Fox and the hound.
When Ariel is crying after the wedding party boat leaves
Trusty is hit by a car in Lady and the tramp
When Dumbo's mum is cradling him in her trump, the song baby mine is playing
Ok I'll stop as I'm starting to bawl like a baby.
I know it's not a movie moment but when Homer says goodbye to his mum again and is sitting on his car looking at the stars, that moment and music always makes me tear up.