What is the hardest metal/alloy in existence?

I would guess that it would be some form of steel, but I am not sure that there is one special one that has the highest hardness. The hardness depends solely on the carbon content, so I would guess that some form of tool steel. The previous answer mentioned 1090, but tool steel can have much higher carbon content and form many more carbides to contribute to hardness. But if you were more concerned with yield or tensile strength, there would be a very different answer.