Lets get this out of the way at the begining. The anime adaptation of One Piece is slow, the padding in some arcs has a real detriment to the enjoyment of the show.

My Score is 89/100 for this reason.

If you want the true One Piece experience then please read the manga. Its faster paced, quicker to get through, and is one of the few manga i'd consider 10/10. The anime still keeps all the amazing aspects of the manga and that is gonna fill up most of this review. One Piece both the manga and anime are special pieces of media that will transcend era and time period. The impact and legacy of One Piece cannot be understated as within the next couple years it will become the most purchased graphic novel in history.

Also,for any terms used that aren't generally common knowledge for anime fans, I will provide a link to the wiki. Now some the explanatio might be quick,and for those I will put an * and explain at the bottom, but I don't want to interupt sections with heavy or long explantions. Any point after the straw hats enter the grandline will also have a spoiler tag on it, but this review will be mostly spoiler free outside of using things as a reference or example.

The adventures of Monkey D. Luffy and the Strawhat Pirates is a modern epic, a tale of found family discovering a vast world, finding new and unique people and cultures, and battling a whole range of characters. The sheer scale of the world Enichiro Oda has created is nothing short of awe inspiring.

But before I spend the next 1000+ words gushing about this series, I have to adress the negatives. I have already adressed the padding and the pace issues it creates, but there are valid criticisms for the anime outside of this.

The Negatives
The early arcs can range from meh to great and its very easy to see why so many people would drop the show before the second saga*. While no arc is bad, some in the anime can be a struggle to get through.

Spoiler, click to view

The long ring long nose arc being a perfect example of how padding can hurt the show. In the manga, this arc is much shorter and more densly filled with comedy. The anime grinds to a hault and while the arc is still funny, it drags on and because it is sandwiched between two other good arcs, it suffers

The animation for the show is another knock against it, while the newest arcs and episodes look amazing, the same cannot be said for most of the show. The overall look of the show has always been good and seeing the animation evolve over 2 decades is a wonder to see unfold. It doesn't change how not great the animation is for the first hald of the show. While the excuse that its an old show works, it doesn't hold up when examining the show as a whole. Toei Animation is infamous for its corner cutting and One Piece is a big victim of this.
Not all arcs are great and enjoyment of them is subjective. The general formula for getting to new islands is usually the same, but things like the villain, the side characters, and the setting can be amazing for some and bad for others. Not every arc will be your favorite.
The show might also just not be for you. There is no perfect product that everyone ever will enjoy. Give the show until like episode 30-40, this is the climax of what is considered to be one of the best early arcs. It really is what makes One Piece amazing and if you don't get into it from there, the show just isn't for you and thats ok.

Side note for this section, unrelated to the material, but the community for One Piece is massive and it means there are tons of toxic fans. If you spend enough time around the show, you will run into one. Please don't let them ruin the show for you.

The Characters
The characters in One Piece are incredible. From the seemingly endless amount of unique designs to the wonderful writing of not only the main cast but most of the villains and side characters, One Piece eccels with its characters. Luffy is one of the best MCs in shonen. Watching him fill up his pirate crew with interesting characters, each with thier own detailed stories and motivations is addictiing. The villains are all fascinating, each serving a different narrative purpose. Some are there to challenge ideals, while others are foils and cautionary tails for Luffy and his crew.
Each Strawhat gets their time to shine, especially early on in the series. Each one grows and evolves with the story. The Strawhats are a family and you really feel that with how they interact with each other, the bicker, make jokes, and enjoy each others prescence. The dynamic of the crew can carry even the most boring or mundane of scenarios. So much of the series is just enjoying watching them be a crew.

The World
The world of One Piece as mentioned before is massive. The world is full and ever changing, with dozens of unique cultures, governments, powers, and races. The world building in One Piece is second to none. The world exists outside of just the strawhats with many large and small factions, world powers, and groups all clashing and interacting. Some of the best moments of the show come after arcs end. Seeing how the strawhats impacted the world and how the world reacts to them makes even the end and in between of arcs just as interesting as the arc itself.
Watching closely and paying attention is often rewarded as small events or minor characters can eventually play a big role in the story. Because of how flushed out the world is, it makes connect with each of the crew's dreams. Their motivations become tangible and you can see the path to achieve them.
The world itself is a character, its full of mysteries and always changing. Power dynamics are challenged and changed. Each action has an impact. The viewer gets sucked in, we want to discover the secrets of the world as much as any of the characters.

Tragedy and Comedy
One PIece can be a very goofy show and comedy is a constant precense. While humor is subjective you learn to love each characters unique brand of humor. Even if not every joke lands it never cuts the tension of scenes or makes the show less enjoyable to watch, One Piece has mastered the balance of comedy and action and drama. Jokes flow seemlessly to action and never gets in the way of the serious moments of the show. Yes, the show is goofy and taking it to seriously can do more harm than good sometimes, but it doesn't mean its just that. One Piece will make you feel every emotion and have you on the highest highs and lowest lows.
One Piece doesn't pull punches with what happens in the series. They make sure you know this by the end of the first couple episodes.

You think you're dealing with saints or something? You're looking at pirates

This quote from episode 4 comes right after someone is shot in the head. One Piece doesn'y glorify death nor does it try to revel in some edgy moral greyness. In the world, people die and stay dead. Drawing a weapon means putting your life on the line.

The themes and Story
While the scale of the story is massive it remains simple, Luffy wants to be the king of the pirates. The narrative when broken down like that is as simple as it is grand. The story is watching this journey and experiencing it with the characters. Never has "its about the journey not the destination" been more accurate. So many people watch the show trying to catch up, but they always burn out. The show is long because everything in it is worth experiencing.
There are dozens of themes in One Piece. The show isn't just mindless adventure, there is depth to the narrative. The main theme of freedom is shown most by Luffy. A force that blows through places, someone who values his own freedom and the freedom of others higher than his own life. Luffy's ideals clash heavily with one of the main atagonstic forces of the show, The Marines. A tyranical force that deals out "Justice" to anyone who goes against them, The dominant force for most of the world.

Spoiler, click to view

The Marines protect and serve an elite class, the celestial dragons. Born into power, they own slaves of all races, look down on everyone, and have anyone they want killed. The hypocrisy of the marines often on full display such as them defending the slave trade and punishing the pirates who broke it up. Fascists control the marines and the marines take away the freedoms of people to enforce the whim of these tyrants. The few likable marines who disagree with the top are held down and shunned. Luffy despises this and many characters throughout the story serve as figures of rebellion.

There is so much meaning to be taken from One Piece and discussions over themes and moments can go on for hours. One Piece being so grand allows it to tackle these issues like systemic opression, racism, classism, prejudice, greed, etc.

The Music and Sounds
The openings range from iconic to ok, but when you have over 20 of them thats bound to happen. The music in the show is powerful, despite using the same couple songs for certain moments, the weight and impact never go away. Like Ichigo's theme in blech or Sorrows and Sadness from Naruto. The Voice acting is top notch, you can't ask for more. (The dub is fine, they miscast a lot of characters and imo the dub makes the show less fun to watch)
The Visuals
As mentioned, the animation get rough at some points, but Oda's art style is pleasing to look at. While some might find cartoonish, its hard to ague that some of the settings and Islands are fantastic to look at. The creativity of the manga shines through in the anime as motion and color is added to the world. Its definitely not perfect but its also not bad. The further you get in the series the more consistent the art becomes and the more stuniing the animation can get. There is also a charm to the older art style and the pre time skip* world.

Personal Enjoyment
I love One Piece. I have been caught up with the anime for almost 3 full years and I still watch it weekly. It becomes part of your life when you get fully into it. Even through the rough parts I love the series. I will be the first to admit the anime isn't as good as the manga. With a new One Piece movie coming out, it will be the first time I get to see One Piece at a theater. One Piece has been my best friends favorite show for years and I gave it a shot after he pestered me to for a year. I am glad its something we now bond over.
Sorry for the length of this review, One Piece is just that big of a series. i could probably talk forever about it, but this is a review not a discussion board. Reviews for One Piece have to be uniquely long and weirdly formatted to grasp the entirity of the show.
In objective terms, one piece could be written a bit better, oda could've not drawn certain things certain ways, the anime is overly padded and maybe more filler would've made the show less slow like it did for naruto and bleach, but thats hindsight talking. I do wish Oda would take more breaks and that the anime studio would skip more weeks. I wouldn't mind it being bi weekly if it means better pacing per episode.

Conclusions
Watch or read One Piece. Its a once in a lifetime property and we'll probably never have something like this ever again. It is worht the time.

*One piece like other shounen is seperated into story arcs, but unique to one piece is that these story arcs fit into larger Sagas.
*One piece has a time skip around the middle of the series (now more like 40-45% through). Its similar to other shonen and media.

Spoiler, click to view

There is a shift in design and tone as well as the characters getting new looks.