Why it rocks:
Amazing animation during dramatic scenes and fight scenes.
Incredibly interesting villains. From Big Mom and her pirate crew, to Kaido and his pirate crew, to Crocodile and his organization Baroque Works, to Blackbeard and his pirate crew, to all of the marines, especially the admirals and fleet admiral; the list just goes on and on.
Excellent soundtrack.
Tons of excellent fight scenes. The fights aren't that interesting early on, as Luffy tended to just one-shot everyone, but later in the story they become much more difficult, much longer, much more interesting and much more epic and with much higher stakes.
Unique art style and character designs.
Outstandingly unique abilities. The Devil Fruits provide for lots of creativity, similar to the Zanpakuto in Bleach or Ninjutsu in Naruto. There's standard abilities, such as elemental control, but there's non standard ones, such as turning into rubber. Also, the power system is incredibly well fleshed out and complex. From Devil Fruits, to Haki, ect.
Good voice providing both Japanese and English (Funimation's dub). As Naoko Takeuchi did with Sailor Moon, the English dub voice actors were handpicked by Eiichiro Oda, the creator of the franchise.
Phenomenal worldbuilding, often the thing that fans praise the most. The world of One Piece is unbelievably huge and vast, with tons of creative creatures, characters, and environments.
The show has some good comedy.
Aside from the anime, the series also has tons of video games, anime movies, speicals, and OVA's, so even if you finish the main story, you can still get sucked into other side stories with these characters.
Lots of plot twists that you won't see coming, but are still foreshadowed, sometimes years in advance.
Luffy gets some character development later on which makes him less naive, although he still resembles the old Luffy.
Lots of dramatic moments that make you feel for the characters, stand outs being Brook, Nami, and Robin.
The story arcs in One Piece also hold lots of variety, from one involving food, (Whole Cake Island), to arcs involving royalty, (Alabasta and Dressrosa), to arcs involving samurai and ninja, (Wano Kuni).
A perfect blend of amazing male and female characters, unlike many of its contemporaries.
While it doesn't happen all the time, there are times when major characters die in the series. The deaths are good to give the story a sense of realism given the subject material, while it doesn't happen so often that it becomes predictable.
Also, when a character dies in this story, they're dead forever.
The villains aren't the only good characters; there are also tons of memorable and likable heroes, including Zoro, Nami, Robin, and Red Haired Shanks, among many, many others.
Exceptional foreshadowing