You can either use most flowers or most of the flowers. *Most of flowers is incorrect.
This Grammar site explains that
When speaking in general, we use most to refer to a quantity of an unspecific group of people or things. Most is a determiner (a quantifier) to the subject noun:
- Most students ask questions.
As for most of the, it says:
When speaking specifically, we use most with a prepositional phrase that limits the number to a specific group:
- Most of the students [PP] ask questions.
Here, most is accompanied by the PP of the students to refer to an understood (known) group of students.
So depending on whether you want to speak unspecifically or specifically, use either most flowers or most of the flowers.