The metal with the highest hardness among steel, iron, and aluminum is steel. Steel is an alloy of iron and carbon, and its hardness can be significantly increased through various heat treatment processes. Iron, while harder than aluminum, does not possess the same level of hardness as steel. Aluminum, being a softer metal, is generally softer than both iron and steel.

The hardness of a metal is a measure of its resistance to indentation or scratching. The Vickers hardness test is a common method used to measure the hardness of metals. According to the Vickers hardness scale, the hardness of steel can range from 200 to 650 Vickers, depending on the type and composition of the steel. In contrast, the hardness of iron typically ranges from 150 to 300 Vickers, and aluminum ranges from 50 to 150 Vickers.

The hardness of steel is primarily due to the presence of carbon atoms in the iron lattice, which disrupt the regular arrangement of atoms and make it more difficult for them to move past each other. This results in a higher resistance to deformation. Additionally, the addition of other elements such as chromium, nickel, and molybdenum can further enhance the hardness of steel through the formation of harder phases or by strengthening the iron lattice.

Iron, while harder than aluminum, lacks the carbon content that allows for the same level of hardness as steel. The absence of carbon limits the ability of iron to form the same hard phases that are present in steel. Aluminum, on the other hand, is a relatively soft metal due to its low melting point and the nature of its crystal structure. The face-centered cubic (FCC) crystal structure of aluminum allows for more movement of atoms, making it easier to deform and less resistant to indentation.

In conclusion, steel has the highest hardness among steel, iron, and aluminum due to its carbon content and the ability to form hard phases through heat treatment. This makes steel suitable for applications requiring high strength and durability, such as in construction, automotive, and aerospace industries.