The best parts of One Piece are its character design and concept art. The set pieces, ships, costuming are whimsical, engaging, and endlessly creative. Visually it prevents the show from falling into the anime trap of having a number of similar characters with different hair colors whose goal is overthrow a generic anime bad guy.
The storytelling and pacing are less creative. I haven’t watched the entire series but 17 years of production and 800 episodes in, and I do not believe they have actually found the One Piece, the treasure that contains everything the world has to offer. Each episode contains about 16 minutes of new animation (if you exclude the theme song, recap and end credits) stretching each arc out into a lengthy set of installments. Monkey D Luffy is an overpowered character, so in order to introduce tension to the story he usually needs to be sidelined from important fights, because he’s asleep or at the bottom of the ocean or something.
You will see every character cry in one piece. This includes the villains but is especially true of the protagonists. Usopp is particularly prone to this, but it comes to everyone in turn. You’ll watch through everyone’s childhood and still lack coherent answers to basic questions (does Luffy have parents? Why was Sanji working on a ship at age 7?) Thats frustrating to me.