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. Everything after E8 felt like a lede for an S2.
What are yours?
Filler arcs, such a common cause of show death.