WELL THINK ABOUT IT
what qualifies something as an ancient weapon?
- something that posseses the power to destroy the world
that is it really........
so the straw hat could be the final and difinitive AW since it indirectly has the power to destroy the world, don't you think ?
it is seen as a symbol,soet of, for the will of D. and it has been passed on for generations to the people who show promise, promise to change the world and possibly destroy it.
when you think of luffy's journey, the straw hat has played a key role in many of the events and sometimes has drastically affected the outcome ......... but what lies at the end of the journey; as predicted by whitebeard and other people the world will be turned upside down and all because of the straw hat . This i think explains why it has been such a central artifact.
but also if you think about it the staw hat is the most powerful of all the AW since it will bring all the other ancient weapons together and possibly "create" on piece whic i think will be formed by uniting all the oceans into "one piece" and even create the legendary ALL BLUE
just a theory