Carried by the world-building and main lore, otherwise below average

I'll make these 2 important points first:

• I think that the manga is so highly rated only because it's mostly rated by those who didn't give-up halfway or didn't bother to attempt it (given its length), so naturally I think its rating is heavly skewed in its favor. There's an interesting statistical analysis article from 2020 on MAL, done by Shymander, where you see at which episodes people drop it or put it on hold (google "One Million Strong A Statistical Breakdown of One Piece").

• I'm giving it a 5/10 because everything other than the main lore and world-building are below average, meaning the characters, the art, the combat, parts of the storytelling, and the general tone (i.e. humor). Despite the relatively low rating, it's a massive compliment when taken into context.

It would have been better as a novel, rather than a manga.

I'll start with the pros and then with the cons, and I'll try to keep it as concise as possible.

Pros:

World-building is absolutely S-tier. The sheer amount of "life" that was bred into the world is insane.

• The amount of races: humans, giants, fish-men, dwarves, skypieans, cyborgs, zombies, toys, harpies, minks, snakemen, etc. etc. The way each race occupies a piece of the story, having their own personal struggles, history and goals. It was even directly emphasized with Big Mom's wish.

• The amount of islands: technically millions but even in the story, so many islands were reached. A lot of them were touched upon in the history, many of which were extremely diverse not only in their populations but also in their personal history that also ties up the the main lore most of the time, leaving hints here and there (i.e. Roger passing through X island and doing Y)

• The way the sea itself is divided: I personally like vertical universes, meaning there are dimensions "above" and "below" (heaven-earth-hell template), howeve One Piece was able to create a massively developed horizontal universe that was enticing (north blue, east blue, south blue, west blue, redline, grandline, new world, ocean world and sky world, etc. etc.) that inspired mystery and fear. Although, I'll admit that the sea divisions didn't go as far as I would have liked. However it's still very fair.

• The emphasis on eras: it's always a plus to have the "ancient" world and the "present" world, constantly going back-and-forth between the two. Withdrawing information from the ancient world to feed the present world's underlying foundation and causes for the way things are today. Even ancient era stuff leaking into the modern world (i.e. constantly hinting at cyborgs).

Lore is fantastic. It turned a seemingly boring little pirate world into a grandiose treasure of ancient history, government conspiracy, massive plot twists and mysteries.

• The way devil fruits were turned into a classed system of "types" (Paramecia, Zoan and Logia) was a great surprise. I originally expected it to be a fairly mundane, but then it expanded and eventually reached the point of awakenings.

• The way haki was also classed (observation, armament, conqueror) and also had its own little hierarchy with conqueror haki being a matter of inheritence (at least as far as I understand). The way it can rival and outright negate devil fruits (what we originally thought would be the end-all of powers), with marking figures like Shanks and Roger.

• The ancient history that's being carried by so many elements, such as historical ancient weapons that can be used to rule the world, oddly placed technology that hinted to the void century. The introduction of poneglyphs, ancient language that can only be deciphered by [few] people, and then road poneglyphs, with the promise of a final island.

• The involvement of government censorship and conspiracy and the introduction of a surprise antagonist (Imu) which I think was probably the most important reveal so far and the way he was introduced after introducing the empty throne. Also the introduction of an anti-government unit that also has its mysterious figure-head AND happens to be the father of the main character. The odd behavior of Shanks, and much more. Simply insane stuff.

• The factions (World Government (+Marines), Celestial Dragons, Pirates, Revolutionary Army, neutral scientists, etc.)) and their sub-divisions (7 Sea Warlords, etc.) create very interesting dynamics and it ties up the world lore very well.

Cons:

Art Style is horrible.

• The character design is absolutely horrendous. Characters look like a VR Chat lobby. Half the people are built like wisdoom teeth and others like bowling pins. It's distracting, especially when there's a serious matter and you have a guy built like a humpty dumpty version of Andre The Giant with lipstick and high-heels staring at you in the pannel. Also everything is round. Faces, eyes, bodies, nothing is well-proportioned and crisp, it always has to have that rounded-out goofy shape.

Characters are lackluster and uninteresting.

• Luffy, the main character, is absolutely horrible. Unlikeable, uninteresting, doesn't drive any kind of moral dilema or emotion forward. He's a headless chicken who's running around, recklessly bringing everyone around him in his troubles. You never feel like he's actually challenged on anything. He appears, doesn't know what's going on, beats the strong guy and on to the next. These 2 quotes in the image below summarize it: "I don't want to hear where the treasure is, I'm not going on a boring adventure!" and "I want a world where my friends can eat as much as they like!!". Awful.

• The Straw Hats as a whole are just a mess. Uninteresting character development (if any) and there's always the annoying Usopp, Nami and Sanji, are all absolutely detestable. The others are just a mess. Zoro could have been something much much better than what we were given. The only one with any value is Robin because of her backstory, her goal and I suppose her age also plays a part in her maturity. The whole crew's character development feels like it just stalled from beginning to end. All their interactions are boring as well. Character development is always hard to put into words because it's something you intuitively catch when watching the characters, so that's as much as I can say without babbling too much.

• There's also way too much unimportant characters that get pannel-time for absolutely no reason other than build upon the goofy moments of the story, which brings me to the next point..

Tone of the story is way way way too goofy to be enjoyable. Every single scene is reduced to an anti-climactic goofy mess.

• Misplaced and cringy humor. Sanji and Usopp are classic characters for that. Funny-face humor all manga long is incredibly tiresome and off-putting, without getting into how terrible the "jokes" are.

We could go on for pages, but that should point out the idea.

• The tone is also reflected by Luffy's character development, previously mentionned. Quotes like "I want a world where my friends eat as much as they want" while he's in Gear 5 beating down Kaido...absolutely anti-climactic.

Fights aren't exciting or thrilling.

• Powers and combat scenes are, again, very goofy and very very cookie-cutter. The worst thing being that Luffy is a rubberman for most of the story and his transformations being random goofy nonsense as well. There are some specific powers that are nice like Law's chambers, but practically no others.

• A very sedated form of violence (2nd pannel below) where statements like "I will annihilate/burn you to ashes" are instead reduced to "It's time to kick his ass!".

Storytelling is very spaced out in terms of main storyline. I can already imagine how the anime must be terrible on this aspect if it's already a problem in manga.

• The side-stories being injected inbetween the main elements of the story are absolutely boring and uninteresting. There's about 20 side-stories going on during Wano's arc and they're all less interesting than the next.

• Every single important pannel needs a 3-pannel comedic relief arc for some reason. It becomes unbearable after a while.

Summary

Pros:

• World-building (10/10)
• Lore (10/10)

Cons:

• Art style (2/10)
• Characters (2/10)
• Tone (2/10)
• Fights (4/10)
• Storytelling (3/10)

Overall:
5/10, would be better as a novel than a manga.