This shall probably sound lame, but, for a movie (and a book, really), it's tied between the end of the Rankin-Bass version of The Hobbit, when Thorin admits his fatal avarice to Bilbo on his deathbed, and the end of Harry Potter & the Goblet of Fire, when Amos Diggory is crying over the corpse of his son, yelling out, "That's my son! That's my boy!"
For a TV show, it's the Wild Thornberrys special, The Origin of Donnie, wherein a flashback shows just how sad the kid's early life was (his biological parents were brutally murdered by poachers for thwarting their attempt to steal a young orangutan). It was too much for my prepubescent mind to handle, and to this day still brings a tear to my eye.