The hardest type of steel is generally considered to be tool steel, specifically those classified as high-speed tool steels. These steels are designed to have exceptional hardness, wear resistance, and heat resistance, making them ideal for cutting tools, drills, and other applications where maintaining sharpness and durability under high temperatures is crucial.

Some common examples of high-speed tool steels include:

1. M2 steel: This is a widely used high-speed tool steel with excellent toughness, wear resistance, and heat resistance. It contains a combination of tungsten, molybdenum, vanadium, and carbon.
2. T1 steel: Another popular high-speed tool steel, T1 steel offers excellent hardness and heat resistance. It consists of tungsten, chromium, vanadium, and carbon.
3. M42 steel: Known for its high wear resistance, M42 steel contains a combination of cobalt, molybdenum, and high levels of tungsten, along with other elements such as chromium and vanadium.

It's important to note that while these tool steels are exceptionally hard, there are other measures of steel performance, such as toughness, corrosion resistance, and other specialized properties, that may be more important in certain applications. The choice of steel will depend on the specific requirements of the intended use.