Vishnuji's knowledge about God and Brahmaji's question
Shrimad Devi Bhagwat Puran, published from Gitapress Gorakhpur, edited by Shri Hanuman Prasad Poddar and Chiman Lal Goswami
Skand 1, Page 28-29
Translation: Naradji is replying to a question asked by Vyas ji and says that the same question was put by his father Brahma Ji to Vishnu Ji.
Brahma Ji was astonished to see Vishnu ji doing meditation and therefore Brahmaji asked Vishnuji that you are the superiormost amongst Gods, then why are you meditating. Brahma ji says that I am really astonished to see you "the ruler of the world" doing meditation.
On this Vishnu ji replies,
"I have to live under the subjugation of Shakti in every respect. I continually remember that Bhagwati Shakti. Brahma Ji, in my knowledge, there is no other God superior to this Bhagwati Shakti."
Essence: the above mentioned lines from Durga Puran give an idea about Brahma's knowledge who considers Vishnu to be the Supreme God, whereas Vishnu ji is considering Durga (Shakti) to be the Supreme God.
Hence, according to Brahma, Vishnu is Supreme God, and according to Vishnu, Durga Ji is Supreme God.