Solids are often the most dense of the four states of matter. Density is a measure of mass per unit volume, and particles in solids are packed tightly together in a regular pattern. The proximity of particles in solids causes a large mass to fill a small volume, resulting in a high density.
In contrast, liquids have a lower density than solids because the particles are less closely packed and have more ability to move relative to one another. Gases have the lowest density because the particles are widely spread apart and fill the entire container, resulting in a low mass per unit volume.
As a result, solids have the maximum density of any form of matter due to their densely packed particles and stable positions.