Indeed, the scriptures and the teachings of Srila Prabhupada emphasize that Visnu-tattva, which refers to the Supreme Personality of Godhead, Lord Vishnu or Lord Krishna, is the highest and topmost among all tattvas. Here are the references provided along with their explanations:
Srimad Bhagavatam (SB 8.3.17, Purport): The viṣṇu-tattva, or the category of Vishnu, is considered all-pervading and supreme among all the tattvas. While there are other tattvas such as jīva-tattva (the individual souls) and śakti-tattva (the divine energy), the viṣṇu-tattva stands above all.
Srimad Bhagavatam (SB 10.2.9, Purport): This verse explains that jīva-tattva (individual souls), viṣṇu-tattva (Supreme Personality of Godhead), and śakti-tattva (divine energy) are all integrated with the Supreme Lord. When Lord Krishna appears, He manifests with all these integrated parts.
Sri Caitanya-caritamrita (CC Adi 7.10, Translation and Purport): Lord Caitanya Mahāprabhu, who is the Supreme Personality of Godhead, is also ecstatically a devotee. He is īśa-tattva, the Supreme Living Force, or Viṣṇu-tattva.
Sri Caitanya-caritamrita (CC Adi 7.15, Translation and Purport): Lord Caitanya Mahāprabhu, Nityānanda Prabhu, and Advaita Prabhu are all manifestations of the Supreme Lord, viṣṇu-tattva. They are worshipable by all living entities, including śakti-tattva and jīva-tattva, represented by Gadādhara and Śrīvāsa.
Srila Prabhupada Lectures: In the lecture on Srimad Bhagavatam (SB 1.5.15), Srila Prabhupada explains that there are different categories of tattvas. Viṣṇu-tattva is the Supreme Absolute Truth, jīva-tattva is the individual souls, and śakti-tattva is the energy of God. The demigods, although powerful, are not viṣṇu-tattva but are counted among the living entities.
These references and explanations support the understanding that Viṣṇu-tattva, the Supreme Personality of Godhead, is considered the highest and topmost among all the tattvas.