There were several queries received about Pūrṇadīkṣā and there is no one standard answer for them really. Every Guru Sampradāya has its own technicalities and peculiarities derived from different pramāṇa granthas.
In most South-Indian lineages (obviously of Srīvidyā), Paraśurāma Kalpasūtra is the primary pramāṇa but this work really does not get into the specifics in terms of which mantra is considered as conferring pūrṇadīkṣā. In practice, Mahāṣoḍaśī and Mahāpādukā are generally considered as constituting the Pūrṇābhiṣeka.
Things are slightly different in the case of Krama Dīkṣā where a lot more technicalities are discussed. This does not mean that the Dākṣiṇātya Sampradāya of Srīvidyā is not a Krama system – even there one progresses through different steps such as Mahāgaṇapati, Bālā, Shyāmalā, Vārāhī, Pañcadaśī, Shoḍaśī, Parā etc. However, in the complex and syncretistic Krama system of the east, each step is greatly delineated. In the case of Pūrṇadīkṣā, which is obtained for all three Mahāvidyās, is the starting point to higher initiations.
The pramāṇa clearly states:
पूर्णाभिषेकः सुन्दर्यां षोडश्यामेव कीर्तितः |
अभावे लघुषोडश्यां अक्षरत्रितयेऽपि च ||
In the case of Tripurasundarī, pūrṇadīkṣā is always accomplished through Shoḍaśī. When this is unavailable, one will have to use Laghuṣoḍaśī (adhama pakṣa) or Saubhāgya ṣoḍaśī (madhyama pakṣa). By this statement, it becomes evident that uttama pakṣa is clearly Mahāṣoḍaśī.
Elsewhere it is said:
राजराजेश्वरी विद्या पञ्चमी परसुन्दरी |
श्रीमहाषोडशी पूर्णा पूर्णदीक्षितपूर्तिदा ||
Pūrṇadīkṣā is attained through:
(a) Rājarājeśvarī (aṣṭādaśī)
(b) Pañcamī
(c) Parāṣoḍaśī
(d) Mahāṣoḍaśī
In the case of Dakśiṇā Kālī, the pūrṇadīkṣā is always through Kāmakalā Kālī of Uttarāmnāya:
तथा कामकला काली दक्षिणायामपि स्मृता |
That said, for every form of Kālī, the final dīkṣā is indeed through Guhyakālī who encompasses both Uttara and ūrdhvāmnāyas:
गुह्यकाली सैव विद्या तत्त्वानां नवकैर्युता |
सप्तप्रेतसमासीना महागुह्येश्वरी तथा ||
Generally, the dīkṣā is obtained through Rāmopāsitā and Bharatopāsitā forms, who is Daśavaktrā. Some advanced and fearless upāsakas also venture into shatavaktrā Guhyakālī but those are rare to find.
Again, there is no single standard answer here, one simply needs to follow the valid pramāṇa adopted by one’s lineage.