SB 12.13.16

nimna-gānāṁ yathā gaṅgā
 devānām acyuto yathā
vaiṣṇavānāṁ yathā śambhuḥ
 purāṇānām idam tathā
Word for word: 
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.
Translation: 
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.