Prabhupāda, starts a lecture in August, 1972 by quoting a verse from the Śiva Purāṇa. To strengthen his argument he cites another verse (ārādhanānāṁ sarveṣāṁ viṣṇor ...) supposedly from the Padma Purāṇa.
There are two kinds of bhāgavata: book Bhāgavata and devotee bhāgavata. So in the Śiva Purāṇa, there was a question by Pārvatī to Lord Śiva. Lord Śiva and Pārvatī, husband and wife. Pārvatī means the material nature. Sṛṣṭi-sthiti-pralaya-sādhana-śaktir ekā chāyeva yasya bhuvanāni bibharti durgā (Bs. 5.44). Durgā-devī.
Durgā-devī is in charge of this material world. It is called durga, durga, just like fort. And the superintendent of this fort is Durgā. You cannot go out of this fort; you are imprisoned. Such Durgā-devī, who is so powerful energy—she can create, annihilate, maintain, sṛṣṭi-sthiti-pralaya-sādhana—she is always sitting by the side of her husband, Lord Śiva, and questioning about spiritual enquiries. Just see. Such powerful deity is also ignorant about spiritual life. So the husband, Lord Śiva, is Vaiṣṇava, and she's always asking, and sitting down underneath a bael tree. She can create so many universes, but for her, no building; only underneath a tree.
So the question was, "Which worship is best?" So Lord Śiva replied, ārādhanānāṁ sarveṣāṁ viṣṇor ārādhanaṁ param (Padma Purāṇa): "My dear Pārvatī, of all methods of worship, Viṣṇu worship is the topmost." There are many demigods' worship, even Lord Śiva's worship, but Lord Śiva says, "Viṣṇu worship is the best." Then he says again, tasmāt parataraṁ devi: "My dear Devi, better than that Viṣṇu worship is tadīyānāṁ aradhanam." Tadīya.
Tadīya means those who are in relationship with Viṣṇu, tadīya. Just like His devotee, tadīya. Tulasī-vṛkṣa. Tulasī is tadīya. Or temple tadīya, preaching tadīya. Tadīyānāṁ samarcanam. This is bhāgavata. So Lord Śiva recommends that "Of all methods of worship, Viṣṇu worship is the best, and better than Viṣṇu worship is to worship His devotee or things in relationship with Him." Tadīyānām, that is bhāgavata.
However, in a previous lecture from 1966, he states the same verse is from Śiva Purāṇa:
So from this verse, Lord Caitanya wants to impress upon us that "Don't be misguided for worshiping different demigods. The ultimate issue—the Viṣṇu is worshipable." Ārādhanānāṁ sarveṣāṁ viṣṇor ārādhanaṁ param. Even in the Śiva Purāṇa it is stated that Pārvatī, the wife of Lord Śiva, she was asking Lord Śiva that "Sir, worship... Which worship is the best?" There are different kinds of worship mentioned in the Vedic literatures. So Lord Śiva replied, ārādhanānāṁ sarveṣāṁ viṣṇor ārādhanaṁ param: "There are different kinds of worship, but the best and the supreme worship is worship of Viṣṇu." Then again he gives more stress, ārādhanānāṁ sarveṣāṁ viṣṇor ārādhanaṁ param, tasmāt parataraṁ devī tadīyānām ārādhanam: "And more than Viṣṇu worship is to worship devotee of Viṣṇu." Tasmāt parataraṁ. Viṣṇu ārādhanam is the best. And better than the best worship is to worship the devotees of Viṣṇu, Vaiṣṇavas. This is recommended in the Śiva Purāṇa.
What is the actual source of this verse? Do both Śiva and Padma Purāṇas contain it?