While riding the train, some girls get into trouble.

Why is One Piece considered the best running anime in Japanese?

This might be just an opinion but it is the sole reason I have for reading the 700 odd chapters (and counting) and re-living the series by watching the anime.
Also, the following contains no spoilers. The Plot This is one of the best parts.
When you start reading the manga/ watching the anime, you might feel that it doesn't deserve the hype it gets from Otakus everywhere.
The plot is just too simple and cunning. There is a legendary pirate, he had a legendary treasure called the "One Piece" and just before he is executed, he reveals he has hidden all his treasure it (literally) at the end of the known world.
The entire anime/manga progresses in various "arcs" corresponding to the hero and his crew's journey to reach the end and get the "One Piece". Sounds cliched? Not really.
As the manga progresses, there is so much absurdity, humor, romance, subtlety, tension, drama and other emotions which you really must see to experience and which simply cannot be put into words. The Hero Enter Monkey D.
Luffy, our child-like, food-crazy, typical "baka" with a ridiculous superpower: His body is rubber. Yes.
That's it. Again, you might wonder "Are people crazy? What can rubber do? This guy sounds worse than Krillin (DragonBall, Z) and all Krillin did was die!" Strangely, you will grow to like him and even come to envy that "silly" power. P.S.
The "D" is not silent.
Again, look out for this letter to enjoy the series even more. The Creator Eiichiro Oda is the mangakka your mangakka wants to be. 1.
He sleeps from 2 - 5 am. 2.
He spends all his waking time drawing manga, doesn't even take a day off to spend time with family.
(One Piece is an ongoing manga since 1997!) 3.
He has served as assistant to Nobuhiro Watsuki, another legend himself (Samurai X, if you haven't seen this, you have missed out on a cult classic) 4.
He got hospitalized due to throat infection and still refuses to stay down churning out a new chapter with the scan-lations popping up weekly! Nuff said. In short, he symbolizes that spirit of his own manga. The Style There are very few manga, no matter when you read them, will give you the same effect always.
The story is honest and straightforward and pacing is very consistent even with the filler arcs as they tend to go back to the story always.
The art-syle is silly at times but it grows on you and stays with you till the end.
The characters appear dense on the surface given their stereotypical shonen style entry but soon each one evolves and not one of them appears to be insignificant even if he is a sub-sub-sub character who had only one cameo appearance.
Very rare to find that type of casting. I could go on forever singing praises about this manga but I cannot do sufficient justice to Oda-san's masterpiece.
One might say that this is a purely biased review but you should not make opinions until you have truly seen this series and compared it to the premier shonen manga of all times.