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"Las mejores" is a phrase which is often translated as "the best ones", and "los" is a definite article which is often translated as "the". Learn more about the difference between "las mejores" and "los" below.
las mejores( 
lahs
meh
-
hoh
-
rehs
)A phrase is a group of words commonly used together (e.g., once upon a time).
a. the best ones 
¿Por qué no llevas esas manzanas? Son las mejores.Why don't you buy these apples? They are the best ones.
a. the best 
Estas son las mejores películas que he visto en mi vida.These are the best movies I have ever seen.
los(
lohs
)A definite article is a word that identifies a noun as specific or known to the speaker (e.g., the girl).
A pronoun is a word that stands in for a noun (e.g., she).