Spain
If you are asking what country has the largest Spanish-speaking population, it's Mexico.
Spain qualifies as such.
Mexico was a colony of Spain.
Spain.
Spain
It was a colony of Spain, but Mexico never was part of Spain's territory.
Spain is a much older country than Mexico. Mexico has declared its independence from Spain in September 16, 1810, winning it in September 27, 1821 and became an independent country since then. Spain on the other hand, began as a monarchy state in 1479, becoming and independent country in 1492.
Spain qualifies as such.
At one time Mexico was a colony of Spain.
Spain
Spain.