When it comes to the strongest steel for a sword, there isn't a definitive answer as it depends on various factors and the specific requirements of the sword. However, there are several steel types that are commonly known for their strength and suitability for swordmaking:

1. High-Carbon Steel: High-carbon steels, such as 1095, 5160, and T10, are renowned for their strength and durability. They have a high carbon content, allowing them to achieve high levels of hardness and edge retention. These steels can withstand heavy cutting tasks and maintain their sharpness well.
2. Tool Steel: Tool steels, such as S7 and D2, are known for their exceptional toughness and wear resistance. They are often used in applications that require strength, such as industrial tools and dies. While less common in traditional swordmaking, they can be considered for specialized purposes.
3. Spring Steel: Spring steels, like 9260 or 5160, are chosen for their excellent combination of strength and flexibility. They are frequently used in the production of functional swords that can withstand impact and bending without permanent deformation.
4. Damascus Steel: Damascus steel, whether traditional or modern, can exhibit a unique combination of strength and aesthetic appeal. While the strength of Damascus steel primarily depends on the specific steel alloys used, the layering and forging techniques can enhance its overall strength and durability.

It's important to note that the choice of steel for a sword should consider the intended use, blade geometry, heat treatment, and the skill of the swordsmith. The overall quality and performance of a sword depend on the interplay of these factors, as well as the specific requirements and preferences of the wielder.