In the "One Piece" series, the strongest sword known so far is the "Kitetsu Meito" called "Shodai Kitetsu." The Kitetsu swords are a set of cursed blades known for their exceptional quality and unique properties. Among them, the Shodai Kitetsu is regarded as the most powerful.

The Shodai Kitetsu is one of the 21 Great Grade swords recognized as Meito, which are famed swords with extraordinary abilities and craftsmanship. It is said to be an extremely sharp and durable blade, capable of cutting through almost anything. However, the sword is also said to carry a curse, as all Kitetsu swords are believed to bring misfortune to their wielders.

It's important to note that "One Piece" is an ongoing series, and new swords or developments may occur beyond my knowledge cutoff in September 2021. The story is continuously evolving, and new powerful swords or revelations could be introduced in the future.