What makes One Piece so great for me is all the small details that Oda puts into it. I mean all the puns, the metaphors, and other small details that makes the series soo special. And of course it's epic story line and good humor. I've never seen an author put so much puns and metaphors into a series, and he does it so effortlessly. Oda doesn't spend so much time on it, nor does he make the small epic details very visible.
For example, in chapter 69 Luffy made a drawing of Nami, one of his crewmate(she had just joined the crew), he thought she was a fishman/mermaid. He drew a really goofy picture of what he imagined she looked like. Almost 600 chapters later, hidden in the background there is a fish-person that looks exactly like the one luffy drew.
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Also the fact that some of the technique names have double names. You don't notice it the first time you watch it. For example Oni Giri, Zoros technique. Was used the first time in chapter 2-3 I think. But I only now, like 12 years later found out that not only does Oni Giri mean: Demon Slash, but it also has a pun like: " The pun in the name is that Onigiri is also the name of a Japanese rice snack." and all the Oni Giri variations:
Yaki Oni Giri - Burning Demon Slash, but it also means: The pun in the name is that "Oni Giri" can also mean "rice ball", and adding "Yaki" to it would change its meaning to "grilled rice ball".
Enbima Yonezu Oni Giri - Charm Demon Sleepless Night Demon Slash - "shrimp mayonnaise rice ball" in Japanese.
There are SOOO many more of those.
Theres a fishman who is name Ikaros:
"His name comes from ika (イカ、烏賊), the Japanese word for "squid". It is also a reference to the Greek mythological figure Ikaros (more commonly known by the Latin spelling of his name, "Icarus").
His giant squid friend, Daidalos, died from getting too close to the sun is also similar to the story of Icarus, who died after his wax wings melted when he flew too close to the sun and drowned in the sea. This comparison is made further by the giant squid's name, a play on Icarus' father, Daedalus."
Ikaros is a squid fishman. Those puns are from last arc alone.
One Piece is a great series in it self, but it is all those small details, puns and metaphors that puts it way above all other series for me.