The usual rule is that superlatives (most, biggest, tallest, longest etc) are preceded by the definite article ('the
'). It is customary to add a qualifier
Mexico City is the
most populous city in the world
Of the countries of the world
, Monaco contains the
Of all the shops in our town/district/country
ours sells the
However, 'most' can mean "the majority of", and in that case 'the' is not used.
Most British people drink tea.
Most people wear clothes.