One Piece

Oda himself confirmed in an interview with Momoko Sakura that the One Piece isn't something like "the journey itself was the real treasure" and that it is, in fact, a physical reward."Hitotsunagi no Daihihō", translating to "The Great Treasure of One Piece") is a legendary treasure said to be of unimaginable value. It was said to be hidden somewhere deep in the Grand Line, presumably on the last island, Raftel, by the late Pirate King Gol D. Roger. Before Gol D. Roger was executed, he announced to the world that this great treasure, could be claimed by anyone worthy enough to reach it, thereby starting the "Great Age of Pirates."

The Mystery of the Great Treasure 
No one except the surviving members of the Roger Pirates actually knows where and what the great treasure is. Given the recurring theme that a treasure is merely something of great personal value (e.g. Luffy's hat), One Piece may be nothing more than an item 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 Arc when Usopp tried to ask Silvers Rayleigh about it. However, Luffy stopped him on the grounds that learning about it from someone else would defeat the purpose of their adventures and that becoming the Pirate King would have little merit if he already knew about the One Piece. After decades of speculations and doubts, the treasure's existence was confirmed by Whitebeard with his last breath. He mentioned that "a grand battle will engulf the entire world" and "the world will be shaken to its core" when One Piece is found.