One Piece combines character development, epic plots, no filler training arcs, dramatic storytelling, a unique drawing style, multiple combined storylines, PIRATES, BOUNTIES, gourmet grub, adventure, a huge storyline, hot girls, fanservice, insane powerups, multiple-dimensional villains & allies and a crazy backstory, which makes it a masterpiece.

The Strawhat Crew is led by Monkey D. Luffy, the crazy kid with the Straw Hat. To know the method of his madness you have to trace back to his freakishly powerful grandfather Vice-Admiral Garp. Luffy's grandfather threw Luffy into a jungle with nothing but the clothes on his back to fend off wild animals and the like, and Luffy had to survive on his own, at the age of five. This kind of abusive training by his grandfather has made Monkey D. Luffy into the hardened warrior he is.

One Piece would be nothing if it had only one-dimensional characters like the cliched Silent Swordsman or Battle-Hardened Warrior that lost his whole family in a battle. Dragonball Z was a freakishly mind-bending experience, but so many episodes are used up telling the viewer about Super Saiyans and OMGOSH THE GUY HAS OVER 9000 POWER LEVEL or SUPER ULTIMEGATRONIC SAIYANS and who's stronger that who. One Piece is nothing like that.

Sure, you hear about oh so-and-so is so much stronger than so-and-so, but you don't see insane powerups and crappy training arcs. All the character training of One Piece characters is done behind the scenes, so you don't have to spend 10 episodes having Naruto train against a waterfall with a thousand other narutos spamming Rasengan everywhere. 

 #2: Crazy Comedy Capers

The funniest thing you'll find in Naruto is probably him turning Sexy no Jutsu for the seven-millionth time. Comedy is rehashed in anime thousands upon hundreds upon tens of times and it grows old. Not in One Piece however... whether it's Luffy activating his Gear-Third powerup and inflicting massive damage on a surrounding structure, completely throttling an opponent or whether it's Luffy turning off his brain and going Gomu Gomu no Baka with his powers... it's all epic!

                

 #3: Complicated villains, story Arcs and their extremely epic endings!

 To know the depth that each story arc in One Piece goes through I need to make the Alabasta Desert City story arc an example. Sir Crocodile as pictured below is the major antagonist in the Alabasta and Baroque Works Storyline. Firstly, he tricked an entire city into selling him flash powder which steals rain from one city and gives it to another, by first generating funds by creating a secret organization, Baroque works.

 

  Secondly, Sir Crocodile tricks the entire city of Alabasta into believing he is one of the "good guys" and Crocodile becomes a national hero. The princess of Alabasta recognizes Crocodile as a villain and goes into hiding as an undercover spy in Baroque Works. Crocodile wants to cause a war between Alabasta and a neighboring country so he uses the flash powder to steal the rain from the country and give it to Alabasta, given that rain is extremely crucial in desert countries. The rain stolen from the country does not return and the people there suffer for years.

  Thirdly, not only does his actions cause a war between the two countries, Crocodile has the princess of Alabasta taken captive, and Crocodile goes to the neighboring country and has his advisors trick the people into believing the people of Alabasta are evil.

Lastly, Crocodile wanted to start the war between the two countries because he wanted to take over Alabasta. Luffy & the Strawhat Crew find out about Crocodile's evil intentions and in a totally epic battle where Luffy is beaten to death three times, Luffy manages to defeat Crocodile, with GOMU GOMU NO STORM, the most freaking epic attack I have ever seen.

 

 GOMU GOMU NO FREAKING STORM!