What physical property is most useful in identifying a unknown substance?

The most useful physical property for identifying an unknown substance is its melting point. This property is characteristic of a substance and can help distinguish it from others, as pure substances have specific melting points. Additionally, comparing the melting point of the unknown with known values can provide a quick and reliable identification method. Other useful physical properties may include boiling point, density, and solubility, but melting point is often the most definitive.