The best unit for expressing the density of a solid depends on the context and the specific solid in question. However, the most common and versatile unit is grams per cubic centimeter (g/cm³) .

Here's why:

* Practical: It's a convenient unit for everyday use and laboratory work.

* Common: It's widely used in scientific literature and textbooks.

* Relatively easy to measure: It's straightforward to determine the mass and volume of a solid in grams and cubic centimeters.

Other units:

* Kilograms per cubic meter (kg/m³) is another common unit, particularly in engineering applications.

* Pounds per cubic foot (lb/ft³) is used in some industries, especially in the United States.

Ultimately, the best unit for expressing the density of a solid is the one that is most appropriate for the specific application.