Which category includes organisms with the highest number of common characteristics?

In the taxonomic hierarchy, as we move from Kingdom down to Species, the number of individuals in the group decreases, but the number of common characteristics shared by those individuals increases.
$$\text{Kingdom}\to \text{Phylum}\to \text{Class}\to \text{Order}\to \text{Family}\to \text{Genus}\to \text{Species}$$
The species is the most specific level and contains organisms that are most similar to each other.