The Bhagavata Purana is considered to be the purest and greatest of all the puranas.
Great Sage Veda Vyasa was credited to had authored Srimad Bhagavatam.
Srimad means "beautiful" or "opulent," and Bhagavatam means "related to God." Hence, Srimad-Bhagavatam can be translated as "The Beautiful Story of God."
It invokes devotion towards Lord Vishnu and his various incarnations, primarily focusing on Krishna since he was the complete incarnation of Lord Vishnu.
The story behind compiling SB as per SB itself is as Vedavyasa felt discontent even after compiling the Vedas, Puranas, Mahabharata and Vedanta-sutra. So he inquired Sage Narada regarding the same then Narada said that "You have not actually broadcast the sublime and spotless glories of the Personality of Godhead. That philosophy which does not satisfy the transcendental senses of the Lord is considered worthless."
So on the advice of Narada, Vyasa meditated and the Bhagavata was manifest to him.
Are there any other references which indicate Bhagavata Purana is the foremost?
Even if people knew that it is the foremost then why do they refer to other puranas?
The Bhagavata Purana declares Lord Vishnu (Narayana) as Para Brahman Supreme Lord who creates unlimited universes and enters each one of them as Lord of Universe.Lord Vishnu engages in creation of 14 worlds within the universe as Brahma when he deliberately accepts rajas guna. Lord Vishnu himself sustains, maintains and preserves the universe as Vishnu when he accepts sattva guna and annihilates the universe at the end of maha-kalpa as Rudra when he accepts tamas guna.
The intense and personal bhakti described in the Bhagavata is directed toward Krishna as Vishnu in human form.
From the meaning of Srimad Bhagavatam itself it is clear that it describes the beautiful story of GOD(which is strictly monotheistic devoted to parabrahman Narayana/krishna) then why some people project their ishtadevata as the supreme or foremost even if they knew Narayana/Krishna is the Supreme?
The transcendental pleasure and completeness which vyasadeva felt after compiling the Bhagavata Purana was not felt after compiling Vishnu,shiva,padma puranas etc. even though they describe the transcendental glories and devotion of the deity to which the purana is dedicated why is it so ?