One Piece

One Piece
One Piece (glossed in print as ひとつなぎの大秘宝), Hitotsunagi no Daihihō, "The Great Treasure of One Piece", but still pronounced Wanpīsu) is a legendary treasure said to be of unimaginable value.

It is said that it was hidden somewhere deep in the Grand Line, presumably in the last island Raftel, by its previous owner, the Pirate King Gol D. Roger.

The Mystery of the Great Treasure

Aside from the Roger Pirates and Whitebeard before his death, no one, save for possibly his crew actually knows what the great treasure is.

Given the recurring theme of treasure being something of great personal value (for example Luffy's hat), One Piece may simply be an item that Gol D. Roger valued. In Buggy's flashback to his days as a member of the Roger Pirates, he discussed with Shanks the meaning of a treasure, and he found that all the crew shared the same views regarding personal value.

The closest the Straw Hat Pirates have ever come to finding out the nature of One Piece was during the Sabaody Archipelago Arcwhen tried to ask Silvers Rayleigh about it. However, Luffy stopped Usopp from discussing the treasure on the grounds that it would defeat the purpose of their adventures, and that becoming the Pirate King would have little merit if he already knew about One Piece.

After constant speculations and doubts, the existence of the treasure was seemingly confirmed by Whitebeard with his last breath. He mentioned that "all of it will be written into history" and "a grand battle that engulfs the entire world" will come when One Piece is found.

source: onepiece.wikia.com

The Pirate King

The Pirate King (海賊王) (called King of the Pirates in the English versions) is the strongest pirate alive and owner of the legendary treasure that was later dubbed as "One Piece".

The term "Pirate King" was originally a simple nickname for Gol D. Roger to distinguish himself among swashbucklers. Upon Roger's execution, the name was romanticized by the people of the One Piece world into an actual title, much in the same way One Piece itself was simply "Roger's treasure" until the Pirate King's death. In the English manga and anime, "Pirate King" clearly marks this distinguishment; although Gol D. Roger was Pirate King, Monkey D. Luffy seeks to become the new Pirate King 22 years later.

Before his death, Pirate King Gol D. Roger hid One Piece. It was rumored to be located at Raftel, the island at the end of the Grand Line. The frenzy over discovering and owning such a massive treasure sparked the "Great Age of Pirates". Until One Piece has been claimed, there will be no current Pirate King.

Until his death, the closest person to becoming the Pirate King was Whitebeard (Edward Newgate). Many people who had met Luffy, such as Coby and Helmeppo, believe that he will be the Pirate King ; Kokoro also addressed Luffy as the Pirate King[4] and Shakuyaku has stated that both she and Silvers Rayleigh were rooting for Luffy to achieve this goal.

The reasons for a person wanting to be the Pirate King vary, people like Blackbeard and Gekko Moriah wish to have it for the power and influence the title would come with, while others like the Yonkou are considered candidates for it because they're extremely powerful and very close to it, but the two shown so far appear to not have any interest in it. And still others like Luffy and Crocodile (prior to the event that caused him to let go of it) wish to become the Pirate King because it is a dream of theirs. Luffy (and Crocodile in his youth) seem to want it for the adventures that are involved with becoming the Pirate King. The Pirate King is said to be the ruler of the sea (Enel asks Luffy about the country the Pirate King rules [after Luffy says that he will be one], Luffy answers this by saying that the Pirate King is the king of the sea, Enel then says that it would be a very interesting title). When talking to Silvers Rayleigh, Luffy said that the Pirate King is the person with the most freedom on the sea.

source: onepiece.wikia.com