Answer by Tanya Austin, still trying to figure out a way to get into Hogwarts:
I think one of the saddest stories in Harry Potter is that of Amos Diggory.
Remember him? The proud father of Triwizard champion Cedric Diggory.
We were introduced to Amos Diggory in the 4th book during the Quidditch World Cup. Amos comes off as being quite amiable but is irrevocably boastful of his son's acheivements. Amos's world revolved around Cedric. He's jubilant when his son is picked to be Hogwarts' champion. You also learn of his little disdain for Harry when Harry is prominently featured in the newspaper while Cedric doesn't get much attention.
All through the Triwizard Tournament Amos is an overconfident and doting father pinning his hopes on his brilliant son until in a moment's time his son is cruelly taken from him.
Amos, whose dream it was to see Cedric go down as the Triwizard champion, sees his son dragged back lifeless after being murdered.
His son did win, as he expected him to, but he lost his life in the bargain. In the movie you can feel Amos' anguish as he laments, "My boy, my boy."
It is heartbreaking because nothing is worse than seeing your child dead in front of your eyes. (Ask Molly Weasley.)
We don't hear from Amos again because it's easy to see that he probably lost his will to live. His world was shattered when his "bright boy" was stripped from him cruelly. Amos, who was in the stands cheering for his son, never got to say goodbye to Cedric. It is quite conceivable that Amos would have been wracked with extreme guilt for sending Cedric off to find eternal glory, thus leading to his untimely death.
In The Cursed Child Amos makes an appearance again many years after the war. He approaches Harry to ask him if he can use the only remaining time turner to go back and save Cedric. When Harry denies this request Amos feels highly affronted and blames Harry for Cedric's death. He is also miffed that Voldemort considered Cedric "a spare", showing that he could not see how anyone could overlook his brilliant son.
Amos definitely comes off as one of those annoyingly boastful parents. But he was a loving father who lost the thing that was most precious to him. So it's justified to say Amos was dealt one of the worst cards anyone can deal with: the death of your child.
Honorable mentions for saddest stories in the HP series - Snape, Sirius and Regulus, Dobby, Kreacher and George Weasley who lost half of himself after the War.
Another top pick would be Colin Creevey. Poor kid. He was kind of a misfit but was unusually bright and cheerful and loyal to his house roots. He was a child when he died, which shows JKR can be brutal too.