This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Pet Sematary
Louis weeping and cradling Gage's deceased body, and when Louis weeps as he carries Rachel's body to the burial ground, hoping she will come Back from the Dead whole and not wrong.
The scene, both in the movie and the book, where Louis and his father-in-law start to fight at Gage's funeral, and knock over his coffin.
The coffin did not actually open and spill Gage's sad, hurt remains out onto the floor for all of them to gawp at, but Louis was sickly aware that they had only been spared that by the way the coffin had fallen—on its bottom instead of its side. [...] Nonetheless in that split instant before the lid slammed shut on its broken latch again, he saw a flash of gray—the suit they had bought to put in the ground around Gage's body. And a bit of pink. Gage's hand, maybe. Sitting there on the floor, Louis put his face in his hands and began to weep.
Gage in general, but the scene where Louis remembers the cap full of blood, then dreams a whole life for Gage through his late teens...and then wakes up. Devastating.
Ellie carrying around Gage's picture, and sitting in his little director's chair. Trying to keep Gage alive, on her Hot 100, you know, in case he comes back...
In the film when Louis has to kill Gage so he can stay dead, and the boy starts crying before childishly going "No fair!" and stumbling until he hits a wall and slumps down dead.
Even worse in the book, where the real Gage comes back for a second and has just enough time to cry "Daddy!" before he dies again.
Also in the film, the look of utter devastation on Louis's face as he injects Gage and then watches his son stumble and then slump down. Oh, God...
To make things even worse, Louis briefly remembers a happier Gage right before he kills his son.
The music cue that plays during this scene (rather appropriately titled "Adieu Gage"), courtesy of Elliot Goldenthal, just drives the whole thing home. Crowning Music of Tearjerking, anyone?
Rachel's memories of Zelda's death in the film, as scary and unnerving as it is is also really tragic because of the circumstances. Here is this little girl home alone with a sister that she subconsciously wished was dead, felt like she would be blamed for not only wishing her sister was gone but for being unable to help her.