SB 1.3 - kṛṣṇas tu bhagavān svayaṁ
All the above mentioned incarnations are either plenary portions or portions of the plenary portions of the Lord, but Lord Śrī Kṛṣṇa is the original Personality of Godhead. All of them appear whenever there is a disturbance created by the atheists, and the Lord appears to protect the theists. All incarnations mentioned and not mentioned here, who are portions of Mahā-Viṣṇu or empowered jīvas, create happiness (mṛdayanti) in the world whenever (yuge yuge) it is afflicted (vyākulam) by the demons (indrāri) and their ideas. But Kṛṣṇa is the ultimate form of Bhagavān. Here Kṛṣṇa is distinguished from other incarnations. He is counted among the avatāras, because He descends from His abode to this world. Sometimes He comes Himself, and sometimes He sends His plenary portions. Originally the Lord is full of all opulences. When they are partly manifested thru the plenary portions or parts of the plenary portions, certain manifestations of His different powers are required for their particular functions.
Some of the aṁśas are Matsya, Kūrma etc, and some of those who are kalā are the Kumāras, Nārada and other āveśāvatāras. Kumāras are invested with knowledge and Nārada is empowered with bhakti. Pṛthu and Paraśurāma are also empowered incarnations, invested with kriyā-śakti (action). At the end of Kali yuga, Vāsudeva enters into Kalki. Some are invested with great powers and are called avatāras, whereas those with less powers such as Marīci are called vibhūtis. All these forms are parts of the first puruṣa described (Mahā-Viṣṇu). Kṛṣṇa, who is Bhagavān, the avatārī, is the source of this puruṣa as explained in 1.3.1, where it is said that Bhagavān accepted the form of the puruṣa.
Lord Paraśurāma and Lord Nṛsiṁha displayed unusual opulence (power) by killing the disobedient kings 21 times and killing the powerful demon Hiraṇyakaśipu respectively. But even the demon was cut into small pieces by the nails of Lord Nṛsiṁha. The Lord’s empowered incarnation Nārada and indirectly empowered Lord Buddha created faith in the mass of people by showing dharma. The incarnations of Rāma and Dhanvantari displayed His fame, and Balarāma, Mohinī and Vāmana displayed His beauty. Dattātreya, Matsya, Kumāra and Kapila exhibited His transcendental knowledge. Nara and Nārāyaṇa exhibited His renunciation. Thus different incarnations manifested different features, but Lord Kṛṣṇa exhibited the complete features of Godhead. And the most extraordinary feature exhibited by Kṛṣṇa was His internal energetic manifestation of His pastimes with the cowherd girls.