No one knows, actually. Think that's part of the draw, finding what One Piece actually is.

[One Piece is the friends we made along the way--] I don't hold much expectation myself, but I have a feelin' Luffy doesn't really care what it is either. It's more about getting there in the first place and becoming the king of the pirates.

[He really can't remember a time that Luffy cared too much about treasure, other than selling it off so that they can buy more food or some shit like that. Nami-san has other things to say, but they'd probably be broke without her.]