Hare Krsna Mahamantra - references
Vedic mantra on the same level as "om", so powerful that it needs (contrary to other mantras) no rsi, devata, nyasa etc.

The mahamantra is mentioned in various scriptures as the only means of deliverance in this age of Kali:

hare krsna hare krsna krsna krsna hare hare
hare rama hare rama rama rama hare hare
iti sodasakam namnam kali-kalmasa-nasanam
natah parataropayah sarva-vedesu drsyate

The sixteen names of the Hare Krsna mahamantra: hare krsna hare krsna krsna krsna hare hare, hare rama hare rama rama rama hare hare destroy all the inauspiciousness of the age of Kali. This is the conclusion of all the Vedas. (Kali-santarana Upanisad from Krsna Yajur Veda)

hare krsna hare krsna krsna krsna hare hare
ratanti halaya vapi te krtartha na samsayah

Hare krsna hare krsna krsna krsna hare hare: Whoever chants this mantra, even neglectfully, will attain the supreme goal of life. Of this there is no doubt. (Agni Purana)

harir eva samaradhyah sarva deve suresvarah
hari nama maha mantrair nasyatpapa pisacakam

All the grievous sins of one who worships Lord Sri Hari, the Lord of all lords, and chants the holy name, the Mahamantra, are removed. (Padma Purana, Svarga Khanda 50.6)

hare krsna hare krsna krsna krsna hare hare
hare rama hare rama rama rama hare hare
sodasaitani namani dvatrimsad varnakani hi
kalau yuge maha-mantrah sammato jivatarane
varjayitva tu namaitad durjanaih parikalpitam
chandobaddham susiddhanta viruddham nabhyaset padam
tarakam brahma-namaitad brahmana gurunadina
kalisantaranadyasu sruti-svadhigatam hareh
praptam sri brahma-sisyena sri naradena dhimata
namaitad-uttamam srauta-paramparyena brahmanah
utsrjyaitan-maha-mantram ye tvanyat kalpitam padam
mahanameti gayanti te sastra-guru langhanah
tattva-virodha-sanprktam tadrsam daurjanam matam
sravatha pariharyam syadatma-hitarthina sada
hare krsna hare krsna krsna krsna hare hare
hare rama hare rama rama rama hare hare

Hare krsna hare krsna krsna krsna hare hare, hare rama hare rama rama rama hare hare: This sixteen-name, thirty-two syllable mantra is the mahamantra in the age of Kali by which all living beings can be delivered. One should never abandon chanting this mahamantra and take to other so-called purificatory processes which are practiced by rascals, or engage in chanting other metrical compositions of the name of Krsna that are against the pure conclusions of the scriptures, or are filled with rasabhasa. About this divinely spiritual mahamantra, which delivers one from material existence, the original guru, Lord Brahma, has said, kali-santararadi srutite, "The srutis have declared this mantra to be the best means of deliverance in the age of Kali". Having all heard this from Brahma, the sons and disciples of Brahma, beginning with Narada, all accepted the Hare Krsna mahamantra and, having meditated on it, attained perfection. (Ananta-samhita)

Ananta-samhita is a Pancaratra agama, part of the pancaratra corpus known collectively among Gaudiya Vaisnavas as the Narada Pancaratra)

Mahamantra is further mentioned in CC 1.7.83 (krsna-nama maha-mantrera), CC 3.9.56 ('hare krsna, hare krsna' kahe avisrama), Narada Pancaratra, etc.

Some object that Sri Caitanya didn't chant it as it is not recorded in CC in full. This refute His contemporary devotees (also see CC 1.7.149p.):

hare krsna krsneti krsneti mukhyan
mahascarya-namavali-siddha-mantran
krpa-murti-caitanya-deva upagitan
kadabhyasya vrndavane syan krtartha

When will my heart become satiated by perfecting the chanting of the Hare Krsna mahamantra in Sri Vrndavana Dhama, which the most merciful Sri Caitanyadeva personally chanted and distributed to the fallen souls out of compassion? This wonderful mahamantra is mixed with the mellows of love and is the chief and the perfection of all other mantras. They are full of spiritual energies and glories. (Sri Vrindavana Mahimamrta by Srila Prabodhananda Sarasvati 17.89)

hare krsna rama nama gana dana karinim
soka moha lobha tapa sarva vigna nasinim
pada padma lubdha bhakta vrnda bhakti dayinim
gaura murtim asu naumi nama sutra dharinim

"He makes the gift of the song of the names 'Hare, Krsna and Rama', and destroys all obstacles such as sorrow, delusion, greed and suffering. He gives the devotional service of Lord Krsna to the multitude of devotees who are eager for the shelter of His lotus feet. I fall down swiftly to offer my prostrated obeisances to the Lord in His golden form, who holds a string of meditation beads." (Srila Sarvabhauma Bhattacarya, Susloka-Satakam 23)

sri-radhar bhave ebe gora avatar
hare krsna nam gaura korila pracar
(Vasudev Ghosh's bhajan "Jaya Jagannatha Sacira-nandana", text 4)

CB Madhya-khanda 23.75-80: (trans. Kusakratha Das)