When it comes to the #1 ranking, only two shows are even possible, due to their having a leg up in length over every alternative contender. Extra length means more entertainment in just absolute terms, but it also gives the show more chances to try out different things and spread their wings. It allows for more difficult, complicated stories to be told than short stories can fit in to their allotted time, and it allows for longer character development over a broader length of time and events. There is basically nothing length doesn't improve.
Pretty Cure and One Piece are the two longest shows of worth in the anime industry. The difference is Pretty Cure resets every 48 or 100 episodes and starts over from the beginning, telling virtually the same story over and over again, whereas One Piece continues forward with just one giant story from beginning to end. This reset feature ends up limiting Pretty Cure to a smaller set of options in terms of storytelling, as is the decision to set Precure in modern day Japan, real world. Having an all-female cast also weakens its ability to tell varied stories about many different things. One Piece has none of these problems. Set in a fantasy world, One Piece can be about anything the author pleases and there's never any contradictions in the plot or setting. One Piece has both male and female characters. Lastly, One Piece never repeats itself. It's a long story, but it's always moving forward in one single direction, towards the end of the journey at the end of the Grand Line.
With this many advantages, it's hard to believe Pretty Cure has been ranked higher than One Piece all this time. But Precure has its own strengths. The art and music are far prettier. Its cast is larger and more attractive. The many different beginnings and endings are all exciting and emotional. But in a sense, the strengths of Precure are shallower than the strengths of One Piece, which relies on pure storytelling over any of these gimmicks. Pretty Cure's charm wears off after the second time through, but One Piece, with its grand and sweeping narrative that pulls you into the story and never lets go, can be watched through any number of times without it ever getting old.
The longer One Piece gets, the stronger it gets, because each and every episode improves every other episode in the series, by somehow connecting to each other through subtle bonds. Pretty Cure doesn't have this feature because it keeps resetting the world every season. This means that One Piece is getting geometrically better with the addition of each episode, whereas Precure is only getting arithmetically better with the episodes it's adding each week.
The current Dressrosa arc is intimately connected with the previous Punk Hazard arc. It goes without saying that these two arcs support each other like two sides of an arch. But what's even more fascinating is how Dressrosa connects back to the Sky Island arc, to the Blackbeard pirates, the Whitebeard War, Saboady Archipelago, Luffy's childhood, etc. The show builds on itself so thoroughly and carefully, that foundations that were laid years ago are suddenly being called upon now to lift up the next segment of the narrative. The ability to admire how everything is connected is a pleasure reserved only to One Piece. Every other show ends too soon for this pleasure to even appear.
Lastly, Precure is too childish to embrace issues of life and death, it avoids tragedy at all costs and keeps up a happy-go-lucky atmosphere. But in One Piece life isn't that easy. Things can and do go wrong, and even good people can die. It feels like One Piece is closer to the heart of life, while Precure is avoiding the truth and sticks only to peripheral orbits in our life experience.
No matter how much longer Precure gets, it can't get much better, because it just keeps repeating itself in theme and content. One Piece, however, has a new chance to wow us every new island it docks in, because everything is completely different each and every time. This is also the critical flaw of Naruto, in that its content tends to loop over and over with only minor deviations from the previous time around. Shikamaru fights the scythe guy twice. Sasuke fights Itachi twice. Naruto and Sasuke fight each other four times. Obito, Madara, and Pain all make basically the same speech and Naruto responds with basically the same speech to them every time. It feels like we ourselves are trapped in a horrible, endless Izanami eye jutsu that means the story can never just end already. I love Naruto, but when it comes to the virtue of its length, it really hasn't made much use of it. Everything after Sasuke left the village has been repetitive and pointless in terms of plot progression. We've been on a treadmill ever since. There's no better symbol of Naruto's repetitive nature than bringing back characters from the dead as zombie ninjas to go talk with everyone one more time, as though we didn't hear everything they had to say the first time around. Zombie ninjas for a zombie series that just won't end isn't exactly what I praise additional length for. If you aren't doing something new with your extra time, then there's no point doing it at all.
One Piece is already the most popular manga of all time. It hardly needs my praise, which is just one drop in the ocean of love this series has been given from the entire world. But what I'd really like to see is the praise of people in the future, who have a chance to view this world wonder from beginning to end, all in one go. When they get to see the ending, and how it wraps up this long yarn, how happy will they be compared to us, who are eternally stuck in the middle? I envy those people most of all. They'll have the whole story in one piece, while all we ever get to look at is shattered, disorganized shards from one angle or another. We can never get the big picture, what it is all leading to, and so our One Piece is only 1/10th as good as what their One Piece will be. When the already #1 story of all time ends and suddenly becomes 10 times as good, just how good will that story be? It's almost unthinkable what kind of praises could be heaped upon it at that time. Perhaps The Princess Bride said it best:
'Let me put it this way: Have you ever heard of Aristotle, Plato, Socrates?'