Mexican
el departamento
Vivo en un departamento chico.
I live in a small apartment.
Apartment
Rioplatense
el departamento
Vivo en un departamento chico.
I live in a small apartment.
Castilian
el piso
Vivo en un piso pequeño.
I live in a small apartment.
Departamento is the usual word for a flat in Mexico and in the River Plate region. In Spain piso is the everyday noun for the same kind of home. Apartamento appears in all three, often for tourist or rental listings.