The Yami Yami no Mi also known as the Darkness Darkness Fruit, could easily be considered the strongest Devil Fruit in the One Piece world. It was eaten by Blackbeard, the infamous pirate who betrayed and attempted to execute Whitebeard in cold blood.
No two Devil Fruits grant the same power, but all Devil Fruit eaters share two weaknesses: The ocean and Sea Prism Stone. All Devil Fruit eaters lose their ability to swim after consuming the Devil Fruit. It's not just a loss of muscle memory; entering a body of water saps them of all energy.