The hardest steel in the world is typically considered to be various forms of ultra-hard, high-speed tool steels or superalloys. Here are a few examples:

1. Tungsten Carbide: Tungsten carbide is an extremely hard and dense material made by combining tungsten and carbon. It is often used as a cutting tool or in wear-resistant applications. Tungsten carbide can have a hardness of up to 9.5 on the Mohs scale, making it one of the hardest materials known.
2. Diamond-Like Carbon (DLC): DLC is a coating that combines carbon with other elements, such as hydrogen or silicon. It can achieve exceptional hardness and wear resistance. DLC coatings are commonly used on cutting tools, watches, and various industrial applications.
3. CPM S110V Steel: CPM S110V is a high-performance stainless steel known for its exceptional wear resistance and edge retention. It belongs to the family of powder metallurgy steels, which are made through a specialized process. CPM S110V has a high volume fraction of hard vanadium carbides, contributing to its hardness.
4. Rhenium-Based Superalloys: Rhenium-based superalloys, such as those used in turbine blades for jet engines, are known for their high-temperature strength and hardness. They often contain a combination of rhenium, tungsten, and other elements to achieve exceptional properties.

It's important to note that hardness is not the only factor to consider when evaluating the performance of a steel. Other properties like toughness, corrosion resistance, machinability, and heat resistance are also critical in specific applications. Furthermore, the choice of steel depends on the intended use, and different steel compositions are optimized for different purposes.