This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Disney World Of War
Penny's death. This event caused the site to go through days of mourning for her and there was a swell of Penny-related artwork and depictions of scenes of the mourning pups when her body was brought back to camp.
The deaths in Captured Kids of War nearly all of them caused heartache.
Scamp, Geno and The Great Prince's deaths most of all.
Sabor's death for Shere Khan. He can barely function afterwards.
Rebecca and Lucky's deaths. This is widely considered to be one of the most emotional and touching threads the site has ever seen.
Mauko's death. Nuka and Dotty were both devestated by the loss and no one on the site could have predicted how brutally the cub would die or how soon after his birth.
Lady's death caused a chain reaction that all but destroyed her already crumbling family.
Biff's death greatly effected Thomas seeing as he was Thomas's first dog