Death Note is a popular answer, and I also agree that its ideal stopping point is…
Death Note
L’s death
Bocchi the Rock is another show that I felt would be at its best if it stopped at episode eight. If the upcoming compilation movie stops there, I may consider it superior to the TV series. It’s not unheard-of. Violet Evergarden’s film-length recap stopped at Episode 10.
I want to see more of Bocchi-chan's rock!
The title drop + roll credits is its peak and climax. I hoped its finale would be amazing. However, episode eight overshadowed episode 12
What are yours?
Naruto (before the time skip) could have just ended after
spoiler
Sasuke left the village and joined Orochimaru
but noooooo, they had to make almost 100 episodes of pure filler…
EDIT: Bleach could also be cut in half and be better off. On second thought, almost all shonen seems to be like this