The strongest material commonly used for springs is typically considered to be music wire, also known as high-carbon steel wire or ASTM A228 wire. Music wire is a high-tensile, high-carbon steel wire known for its exceptional strength and durability. It has a carbon content typically ranging from 0.70% to 1.00%, making it significantly stronger than many other spring materials.

Music wire is widely used in applications that require springs to withstand high loads, resist deformation, and exhibit excellent spring properties. It is commonly employed in industrial machinery, automotive components, firearms, medical devices, and various other applications that demand strong and reliable springs.

While music wire is often the go-to choice for high-strength springs, it's worth noting that other materials, such as stainless steel alloys, titanium alloys, and certain non-metallic materials like fiberglass or carbon fiber composites, can also be used to produce springs with specific properties tailored to unique applications. The choice of the strongest material for springs depends on factors such as the required strength, environmental conditions, cost, and other considerations specific to the intended use of the spring.