Meilleur comes from bon. Like how in English you would not say “more good,” in French, you can’t say ❌ plus bon. Also, bon is one of these adjectives that come before the noun, and with the superlative you preserve this order. Compare the following sentences we saw above:
noun
C’est le restaurant le moins cher du quartier.
It is the cheapest restaurant in the neighborhood.
vs.
noun
C’est la meilleure boulangerie de la ville !
It is the best bakery in town!
With meilleur, the superlative la meilleure(the best) comes before the noun la boulangerie(bakery), whereas the superlative expression le moins cher(the cheapest) comes after the noun le restaurant(restaurant).