It really depends on what you're looking for and how well the intended elements are executed.
One Piece is a grand epic. There's no manga that is as large and interconnected as it is. Even manga with galactic level scope feel smaller than the world One Piece has crafted, and for that, its lore and world building are unsurpassed. Bare in mind, this was thought up by only one man. Even if you aren't fond of One Piece, what Oda managed to do is commendable.
What I look for in a manga most are rich character dynamics and growth. The Strawhats have typically been my main reason for reading the manga, and seeing how they've all changed and overcome throughout their journey is rewarding. I'm motivated to continue the quest with them. The well crafted world is icing on the cake, but it would mean little to me if I didn't have an engaging cast of characters to follow. And there is why One Piece is so great to me, these amazing characters we got to venture with are but a piece in this vast world. There is far more to it, and if Oda decided to shift the perspective to another set of characters, like the marines, there's enough potential there to tell interesting stuff on their end, because One Piece has active and interesting characters almost everywhere.
However, for as great as many characters in One Piece are, I still prefer the work done in my two favorite mangas, Rurouni Kenshin and Full Metal Alchemist.
The character work, themes, smaller and more focus cast made it easier for me to connect with them on an emotional level. And they still provide a healthy dose of world building, juxtapose in some real world history, and how those real events affect the characters presented in the story.