One Piece is an easy Masterpiece IMO .

Story/Plot -

One piece has the best plot I have seen , it includes some outstanding story arcs like Marineford , Enies Lobby and Wano . It is surprisingly very consistent with its quality too . It has other elements like Mystery which adds intrigue . One Piece has the best mysteries in fiction like the one piece itself , Will of D , Vegapunk ,etc .

Characters -

One Piece has a top tier protagonist who is Monkey D. Luffy . Luffy is a very complex character and a very well fleshed out one too . I have heard people argue he has no development but the only thing I want to tell them is that he doesn't need development because he already is the Perfect Protagonist for his narrative and any further experiments with his character will most likely make him one of the worst characters . One piece might have the best side cast in anime/manga because of many reasons . First reason is the the how each character has back story and motivations which adds to their depth . Second reason is how geniusly Sir Oda handles all characters , example Bonclay , we never thought he would be further relevant to the story but he was , One Piece has 900+ characters and handling all those characters is Nothing short of a Miracle . One piece has a good set of Antagonists but they only need more depth to their characters by showing us their goals and motivations which I think they will definitely get by the end of the story . One Piece has great antagonists like Doflamingo and Katakuri too.

Worldbuilding -

The Lore of One piece is huge . One Piece is considered the King of Worldbuilding in Anime/Manga . It has Awesome theories on its mysteries . One Piece’s History is a Mystery too because of the Void Century and the rest of the History unknown to us currently . It has great Geography too .

Theme -

There exists several clearly outlined major themes in the series-Dreams,honor,freedom,friendship,ambition,adventure etc.The series handles these core themes of the story with fine elegance and nuance.The major theme of One piece highlighted in almost every arc is dreams.The dreams being a motivational factor for each straw hats and not only them but even the antagonists and minor characters in the story have their moment of realization and a feeling of pursuing their dreams.Almost every arc in One piece is the stepping stone for the characters pursuing their dreams with ‘the power of freindship’.Many people diregard One Piece as childish cause of the cartoony eccentric artstyle in addition to the comedy the series beautifully presents.But underneath the surface, One Piece is an extremely dark manga with lots of untense themes.Slavery,racism,discrimination,oppresion,hierarchy and an insight to the dark side of human nature.You’d laugh the most and cry the most watching One Piece and you think this is childish, hah.Oda’s artstyle that hides the context beneath the surface so well it’s the reason why it appeals to all ages.Kids can laugh and enjoy to their heart content of how silly it is,teenagers can have some phenomenal action,and adult folks can find the deeper meaning behind the story.