Gomu Gomu no Mi. It gives the user long-range attacks and incredible speed. And it can sync with almost any other paramecia.
Actually the most powerful fruits shown so far are the logia class (admirals, ace, smoker, especially blackbeard) or the fruits of the Whitebeard pirates, namely Whitebeard himself who has the power to destroy the world.
But it is said Zoan types are the most powerful because they can always get stronger.