Final answer:
The most suitable outgroup species for a cladogram is one that diverged early from the common ancestor, used to determine character state polarity and ancestral traits.
Step-by-step explanation:
The most suitable outgroup species for a cladogram relative to all other species would be one that diverged before the lineage containing the group(s) of interest. This is because outgroup comparison helps in determining the direction of character state changes, known as the polarity, and in establishing the ancestral traits shared by the ingroup.
To construct a phylogenetic tree or cladogram, one compares the shared characteristics of the ingroup species against those of the outgroup. Therefore, a suitable outgroup is typically more distantly related and has diverged earlier from the common ancestor than the species in the ingroup.