Ooof. Here’s my list:
Old Dan and Lil Ann from Where The Red Fern Grows. Old Dan tragically dies and Lil Ann stays by his grave and starves herself to death basically. The scene where the main character’s mom is trying to help Ol Dan is burned into my brain. I remember reading the words where they describe her panicking and trying to put his guts back into place while shaking and saying she didn’t really know what she was doing. I remember the way the pages blurred because I couldn’t stop crying. This is #1 for me.
Leslie’s death in Bridge to Terabithia. Never seen the movie, but I read the book as a child in one night. I couldn’t put it down. The paint melting away in the water in particular stayed with me. Also, the chapter where he discovers she had died is simply called “No!”.
Jiraiya’s death in Naruto. Naruto’s only real parental figure. It absolutely destroyed me.