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?
I am currently on episode 25 of Death Note, and I’m unsure about why I should continue. This is why I agree with you.
The same goes for SAO. After watching the first season, I don’t see any reason to continue. It has a perfectly fine plot, which, in my opinion, doesn’t need any extension.
However, I’m still happy for fans of both of these shows that they got more. At the moment, I’m content with not continuing either. They may not have a proper ending for me, but it’s not like I’m unsatisfied.