Secret about "one piece" what is really?

Everyone thinks that one piece is a treasure but I think it more than that...May be a letter in which it is written something like "Thank you, to who discovered the one piece. The true treasure is to have lived all this adventures. One piece is in your heart" something like that...Or, The Goat! It is who gives the most mystery in One Piece, so may be the one piece. xd

Personally I would say that one piece is what the crew search :

Luffy : the title of Pirate King
Zorro : Become the best swordsman in the world
Nami : Make a world map
Sanji : Find the legendary All-Blue Sea
Chopper : Cure all diseases worldwide
Franky : Build the boat that will travel all seas
Ussop : Become a brave warrior of the sea
Nico Robin : Find the Rio Poneglyphe
Brook : find Laboon

I really do not think that one piece is a treasure. And you what do you think?