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