The characters. So many cool groups and factions with at least some stand out members. Makes the arcs where Oda has tons of different factions extra satisfying like Marineford for instance. I also like how even with little background info, they can leave an impact like somone like Jozu or Vista, and leave you hoping they make a return. Of course the more important characters too.
Devil fruits are a cool system with great abilities (Luffy's, Ace's, Kuma's Marco's and so many more) and I hope we see more logias and maybe another special paramecia beyond Katakuri's. I also think some of the fightimng styles that aren't necessarily linked to fruits, but can be amplified by them, are cool. Rokushiki is great, black leg style is terrific, Zoro's moveset will always be cool and so on.
I also think it's pretty great how Oda always goes all out with designing the islands after coming up with a cool concept. Whole Cake Island was just amazing, Egghead was cool and even Elbaph is shaping up to be great in that regard.
Basically, the detail Oda puts island to island, the large cast of characters with notable additions to the series and the fighting styles and power system of devil fruits. Of course the relationships between characters too like Luffy/Vivi, Luffy/Zoro, Luffy/Sanji, Zoro/Sanji, Usopp with Luffy, Zoro, Sanji back when he gave us great moments like in Water 7 and Enies Lobby. One Piece isn't as consistent as it once was but I'll appreciate it's greatness.