Before a singular count noun modified by 'most' plus an adjective you need a determiner, and this can include either the definite article or indefinite article. So technically, one could say both
She is the most beautiful girl.
She is a most beautiful girl.
For the difference in usage, see a most talented writer -- grammar -- why not "the most talented writer"?.
For an adjective only, without any following noun, you use most by itself, because there is no noun.
Most beautiful was the girl I knew in high school.
The girl next door was most beautiful.
Most incredible is how many...
We have to be careful because in some situations, the noun can be ellided or implied. It should be clear from context if this is the case:
The girl next door was the most beautiful (girl).
The most astonishing (fact) is that zombies are real.
Curiously, to use an implied or ellided noun with the indefinite article seems doubtful as to its grammaticality.
?The girl next door was a most beautiful.
The question mark (?) marks this sentence as having questionable grammaticality. That is not all people will judge it to be grammatical. Thinking about it, I can think of contexts where it would be grammatical, but it would be rare.