The best samurai swords, traditionally known as katana, were typically made using a type of carbon steel known as tamahagane. Tamahagane is a specific type of steel produced through a traditional smelting process in Japan.

Tamahagane steel is created by smelting iron sand (satetsu) with charcoal in a clay furnace called a tatara. The resulting steel has a relatively low carbon content but contains impurities that contribute to its unique grain structure and strength.

The forging process of tamahagane involves multiple cycles of heating, folding, and hammering to remove impurities, refine the grain structure, and create the desired shape and curvature of the katana blade. The blade is then differentially hardened to achieve a distinct hard edge (yakiba) and a softer, more flexible spine (mune).

The use of tamahagane steel in samurai swords provided several advantages:

1. Strength and Durability: Tamahagane steel offers excellent strength and durability, allowing the katana to withstand the rigors of combat and cutting. The forging process and the resulting grain structure contribute to its resilience.
2. Sharpness and Edge Retention: Tamahagane katana blades can achieve exceptional sharpness and retain their edge for an extended period, providing efficient cutting ability. The carbon content and heat treatment contribute to the blade's sharpness and hardness.
3. Flexibility: Tamahagane blades possess a desirable balance of hardness and flexibility. This flexibility allows the blade to absorb and distribute impact forces, reducing the risk of breakage.

While tamahagane steel is highly regarded for its qualities, it is important to note that traditional katana-making methods and the production of tamahagane steel are labor-intensive and require a high level of skill and expertise. Today, modern steels and alloys are also used in the production of functional and decorative katana swords, offering different properties and performance characteristics.

When considering the best samurai sword, it is essential to evaluate factors such as the quality of materials, craftsmanship, heat treatment, and overall design. Expert swordsmiths and reputable manufacturers employ various steels and techniques to create high-quality katana swords that meet specific requirements for strength, sharpness, durability, and balance.