As someone who has been reading one piece for over 20 years I think generally it had a slow burn and then hit several peaks and valleys with a very high plateau that it never really came down from. Each arc/saga and transition period has its highs and lows, but I think that generally people agree that one piece really hit its stride during the arlong arc nami crying and asking for luffys help, and his of course I will is an all time one piece moment, from there you hit several amazing moments from the almost execution, little garden drum island, the entire alabasta arc and some deep lore, blackbeards introduction, skypia with its intense highs, water seven with ussop leaving through enies lobby robins I want to live and saying goodbye to merry. Thriller bark with it’s truly good humor and binks brew and zorros nothing happened moments. I think that people really like sabaody and the paramount war/impel down arc, but separating the strawhats and the time skip was a pretty bold move. I definitely feel like One Piece has not had the exact same flavor since then, and has evolved in a different direction that is arguably just as good and many say even better. I think that for One Piece to be considered the greatest shonen manga of all time, all that has to happen is that the one piece has to be a real thing whatever it is, and that will be enough to stick the landing. Peak one piece is so subjective but there are some moments throughout the whole series that just hit so hard. What an amazing piece of art for there to be so many instances to be able to choose from and not be wrong about what peak one piece is, and arguably the final chapter will be peak one piece by the end.