Originally Posted by diegonel
Both phrases are equally correct and it is simply a matter of choice as to which you would use.
EX. 1. Key West is the southernmost city in the United States.
2. Key West is the most southern city in the U.S.
Not a thing wrong with either sentence so take your pick but I would guess that "southernmost" is the choice of most native speakers.
Southernmost clearly indicates the city's geographical position.
However, while "most southern" certainly can mean a geographical position it can also refer to the city's cultural nature, i.e., it's southern heritage and customs.
To explain my meaning for those who are not that familiar with the U.S. there is a distinct cultural difference between Deep South states and the rest of the country which lingers from pre-Civil War days so it might be said that a city in a southern state like Georgia or Mississippi is "very southern" or "most southern" or "the most southern" in its customs and attitudes.