nimna-gānām - of rivers flowing down to the sea; yathā - as; gaṅgā - the Ganges; devānām - of all deities; acyutaḥ - the infallible Supreme Personality of Godhead; yathā - as; vaiṣṇavānām - of devotees of Lord Viṣṇu; yathā - as; śambhuḥ - Śiva; purāṇānām - of Purāṇas; idam - this; tathā - similarly.

Just as the Gaṅgā is the greatest of all rivers, Lord Acyuta the supreme among deities and Lord Śambhu [Śiva] the greatest of Vaiṣṇavas, so Śrīmad-Bhāgavatam is the greatest of all Purāṇas.