The Fox and the Hound is the saddest movie ever!
Spoiler:
The beginning when Todd's momma dies is really sad, and the part where the Widow Tweed has to leave Todd out in the forest is heartbreaking!
All I have to do is think of the music, and I'm like, "...Soooo saaaaad!!" I can't really think of anything else right now that's really sad... Although everything the Nostalgia Critic covered in his top 11 saddest nostalgic moments has gotta be up there. Anyhoo...

Wuv, Yer mom