This thread is about what Oda has done with One Piece in order to make it as good as it is. It isn't about single moments as much it is about the general techniques, attitudes, style and other such elements that have gone in to One Piece to make it great. This is about how One Piece has been written and drawn in order to achieve its quality as well as Oda's writing style in general and the elements of One Piece's story such as its themes or settings and what they add to the story
In my opinion, there are a few major features that have allowed Oda to create such a compelling manga-The passion and soul going into it, the planning behind it, and the fact that it has a clear identity
When you read One Piece or hear an interview with Oda, you can tell how passionate he is about his work. He clearly loves what he's doing and is willing to put a lot of effort into making what he does great. Furthermore, his characters and messages resonate because of this passion- you can feel that Oda really believes strongly in the values and ideas he puts into. He allows characters to have a life of their own which is what makes people care so much about them. His what allows him to put so much emotion into events like the death of Merry. Because Oda cares so much about the death of a fictional ship, we as readers become invested
Oda's planning also allows him to maintain control of a series where the characters are so full of life. Because he knows where he wants to go with the series, having planned the ending since the beginning and major plot points long before they happened, he is able to make sure everything builds up to the big moments. This also means that even though I'm sure he has to work week to week like any other mangaka in a weekly manga, he's still able to keep things going on a set track most of the time and doesn't deal with deus ex machinas and the like as often.
Finally, Oda knows exactly what One Piece is and who he's writing it for. Oda writes strictly for the shonen demographic, and has specifically said he writes based on what he feels his 15 year old self would find cool. He isn't writing for grown men, for girls, for women. He's writing strictly for teenage guys. But I feel this devotion to a single demographic is actually what helps give One Piece such strong multiple demographic appeal. It's focused and has a strong identity instead of being all over the place by appealing to too many groups of people. By appealing to a very specific part of us, Oda draws a stronger emotional response than he might by forcing in conflicting parts for multiple demographics. He keeps a specific part of us entertained throughout, rather than making us constantly flip between the different sides of ourself