Death has quite the strange track record in anime. Sometimes, death doesn't matter whatsoever; in other cases, death is merely an annoyance.
Fundamentally, death always has meaning, whether for the story or characters; it can draw out the rawest emotion for an anime's inhabitants and audience, bringing with it great consequences for the story.
The fact remains that certain anime deaths bring forth a greater emotional response than others. Why is this? There can be many factors, but shock, surprise, deaths that catch an engaged audience off guard can have the greatest impact. Your Lie In April, you were too obvious with your foreshadowing to be on this list, sorry!
Characters that you thought were simply too cool, too strong to die can suddenly find themselves face down in a pool of their own blood; this can be what makes an anime so exciting, the fear of never knowing if a hero will die before his time. However, unlike the grim realism of Game of Thrones, the fact that it is so hard to die in certain anime and so easy to be revived can also make those instances of final death all the more potent.
10. Demon Slayer - Rengoku
The Flame Hashira Rengoku does not have much time in the spotlight, yet he quickly became a fan favorite. Given this very fact alone, it will be surprising for anyone that such a character dies so quickly, without several seasons' worth of character development.
Even more so, given that Demon Slayer is very much a traditional shounen. Most main cast deaths only occur after a significant time has passed, giving the audience time to fully understand and appreciate them.
Rengoku is pivotal as the Flame Hashira to Tanjiro's search for information on his family's Dance of the Fire God technique, as he is the only lead Tanjiro had. Thankfully, he manages to impart another lead with his dying breath.
Even more shocking is how such a powerful demon slayer dies fighting a demon, the Upper Moon Three, Akaza; everything he has displayed up until this point is complete confidence, overwhelming power, and complete mastery over his abilities due to excessive training.
Rengoku's resolute spirit, his dedication to protecting his juniors as a Hashira should, and his will to never despair leaves a mark on many people who have just grown to like his character.