Are we allowing the anime only powers to be used? Because if we are Samehada has a contender for most powerful swordsmen blade.
"In the anime, they allow the user to transform and manipulate lightning freely. They also boost the user's attack power to the highest limit, which allows them to perform high-level Lightning Release techniques with a minimal cost of chakra. These blades can channel the user's own respective lightning techniques or conduct natural lightning. As long as there is natural lighting to access, the user can battle continuously without tiring and launch lightning from the sky to strike anywhere within a large area. They can also be connected at the hilts to form a larger double-bladed sword. They are able to perform various long-distance, powerful, and defensive techniques, either lightning-based or imbued with it. Even without lightning, they are very dangerous in battle, able to be used in combination with agile spinning movements, sweeping through the targets while the user also rotates."
This is another one of those kinds of arguments where I can see people say no just because they will never have the swords. Personally I think it should be changed slightly, take out the ability to control natural lightning because then you'd win every fight in two posts "make storms clouds, lightning strike." but keep the ability to manipulate chakra created lightning, so you could counter opponents attacks and throw them back at them.