Yamato is a very powerful One Piece character but a very strong case can be made for Luffy being stronger still. His speed and abilities all appear to be greater than that of Yamato’s. In addition, Luffy’s devil fruit ability is just way too versatile to be beaten. He can shift gears at will and while I see this as an extremely close battle right now, it will probably become far less so as the years go on. Luffy will continue to get stronger. Luffy wins.