Determining the most valuable Japanese sword is a complex matter that depends on several factors, including historical significance, craftsmanship, condition, provenance, and the presence of notable signatures or marks.

There are several exceptionally valuable and famous Japanese swords that have achieved legendary status. Some notable examples include:

1. Honjo Masamune: The Honjo Masamune is considered one of the most renowned and revered swords in Japanese history. It is a national treasure of Japan and is known for its exquisite craftsmanship and historical importance.
2. Dojigiri Yasutsuna: Dojigiri Yasutsuna is an ancient sword with a rich history and an exceptional reputation. It is considered one of the "Five Swords Under Heaven" and is highly prized by collectors and enthusiasts.
3. Kogarasu Maru: Kogarasu Maru is an ancient sword with a unique and distinct shape. It is famous for its beautiful blade and historical significance.
4. Fudo Masamune: The Fudo Masamune is another highly regarded sword, known for its exceptional craftsmanship and beauty. It is considered one of the most outstanding works by the Masamune school of swordsmiths.

It is worth noting that the value of these swords is not solely based on their historical significance or craftsmanship. Rarity, condition, and the demand within the collector market can also impact their value significantly. The value of Japanese swords can reach millions of dollars, and transactions involving such high-value swords are rare.

The actual value of a Japanese sword is ultimately determined by a combination of these factors and the interest and willingness of collectors or buyers to acquire them.