What the relative superlative does
The relative superlative singles out the member of a group that has a quality the most or the least: the tallest, the least expensive. English chooses between an ending (tallest) and a word (the most expensive). Spanish has one shape only: the definite article + más (most) or menos (least) + adjective.
English has one invariable the. Spanish has four, and the article and the adjective take their gender and number from the thing described. más and menos never change. There is no word for English one(s): los más baratos already means the cheapest ones.
Juan es el más alto.
Juan is the tallest.
Esta es la menos cara.
This one is the least expensive.
Estos zapatos son los más baratos.
These shoes are the cheapest.
With a noun
If you name the noun, it goes right after the article. más / menos and the adjective come after it. English says the tallest boy, with the superlative first; Spanish puts the noun first: el chico más alto, not el más alto chico. The article is written once only.
Juan es el chico más alto.
Juan is the tallest boy.
Es la casa menos cara.
It's the least expensive house.
Naming the group with de
The group you are picking from is introduced by de, where English says in: the fastest in the class, the biggest city in Spain.
Ana es la más rápida de la clase.
Ana is the fastest in the class.
Madrid es la ciudad más grande de España.
Madrid is the biggest city in Spain.
Mi abuela es la más simpática del mundo.
My grandmother is the nicest in the world.
mejor, peor, mayor, menor
The special comparative forms become superlatives with the article, with no más, just like English best and worst: el mejor, la peor, los mayores. mejor and peor go before the noun, as in English. mayor and menor are about age.
María es la mejor estudiante de la clase.
María is the best student in the class.
Es la peor película del año.
It's the worst film of the year.
Pedro es el mayor de los hermanos.
Pedro is the oldest of the siblings.
