What is the difference between 'profesor' and 'maestro'?
Which is the most common way of saying teacher?
It depends on country. Both are synonyms and understood everywhere. However, "maestro" can be old-fashioned in some countries, but not in Mexico. So, if you are in Mexico or talk to Mexican people, use "maestro" with total confidence.
"Maestro" is especially used for primary teachers, but it also means "very skilled person in a field", like in "un maestro de la pintura" (a master of painting) or (it depends on country) "carpenter", "plummer" and other manual occupations that in past times required a life of training and had "masters", "prentices" and similar categories.
Maestro = teacher
Profesor = professor
Generally speaking, maestro is in primary school and profesor is in university and sometimes high school, but this varies from country to country. "Maestro" can also mean "master" when related to your trade, as in master carpenter.