One Piece is the pinnacle of battle manga, taking the genre to it's utmost potential. The story is magnificent and ever-expanding, the world captivating, the characters lovable and most of all the journey is one hell of an adventure. For some, it's a bit slow paced. The anime definitely suffers a lot from fillers and dragged out episodes, but not the manga. While every chapter isn't vital to the overarching story, it does something to make you more familiar with a certain character or the world in general.
Oda has himself said that the story is just getting more and more meat, and that there is so much in the universe he wants to include. Literally sacrificing himself to make this manga it's best, he sleeps only 2-3 hours at night because it makes him come up with better ideas. His designs are unique and you never know where he will take the story.
I've read Naruto, Bleach, Fairy Tail just to mention the other similar large ones, but they always miss out on a few points. For Naruto, the plot is too predictable and goodie-goodie (too much talk no jutsu for example). Bleach got way out of hand in scale and ridiculousness as it went on, even if it started out great. Fairy Tail has no character development, and while the art is some of the best out there, it seems to be going nowhere (Naruto and One Piece in particular have a clear end-point which becomes clear after you read some volumes).
All in all, One Piece is deserving of being the best-selling manga, and is one of the best manga out there even among the shorter manga like Shingeki no Kyoujin and Fullmetal Alchemist.