Principal, Most Principal and Topmost Principal Holy Names

(I) In the Brhad-visnu-sahasranama-stotra, Uttara-khanda, Padma Purana (72.335) it is said:

rama rameti rameti, rame rame manorame;
sahasra-namabhis tulyam, rama-nama varanane.

Lord Siva addressed his wife, Durga: "O Varanana (lovely-faced woman), I chant the holy name of Rama, Rama, Rama and thus constantly enjoy this beautiful sound. This holy name of Ramacandra is equal to one thousand holy names of Lord Vishnu (Vishnu-sahasra-nama-stotram)."

(II) In the Padma Purana it its mentioned:

visnor ekaika namapi, sarva vedadhikam matam
tadrk nama sahasrena, rama nama samasmrtam

"Each Name of Lord Vishnu is greater and more potent than the entire Vedas combined together, and Lord Rama's Name alone is more superior that a thousand Names of Lord Vishnu."

(III) In the Brahmanda Purana it is stated:

sahasra-namnam punyanam, trir-avrttya tu yat phalam;
ekavrttya tu krishnasya, namaikam tat prayacchati.

"And the pious results (punya) achieved by chanting the thousand holy names of Vishnu (Vishnu-sahasra-nama-stotram) three times or the Holy Name of Rama thrice can be attained by only one utterance of the Holy Name of Krishna."

(IV) Prabhasa Khanda of Padma Purana states:

namnam mukhyataram nama, krishnakhyam me parantapa;
prayascittam asesenam papanam mocakam param.

"O Arjuna, the Holy of Krishna is the most principal (mukhyataram) among all Names of God. One can atone for unlimited sinful deeds by chanting the Name of Krishna which is the supreme destroyer of all sins."

(V) Finally in the Brahma-Rahasya it is stated:

krishna-caitanyeti nama, mukhyan mukhyatamam prabhoh;
helaya sakrd uccarya, sarva-nama-phalam labhet.

"The Holy Name Krishna Chaitanya or Gauranga is the topmost or the most principal (mukhyan) even among all the important primary and most principal (mukhyatamam) Holy Names of the Lord Krishna. By chanting this name just once, a person attains the spiritual results (punya) of chanting all the other holy names of the Lord."

In the Vishnu-yamala it is said:

krishna-chaitanya-namna ye, kirtayanti sakrn narah;
nanaparadha-muktas te, punanti sakalam jagat.

"Those who chant the name of Krishna Chaitanya or Gauranga even once, become free from all offenses and purify all the worlds by their very presence."

Shrila Jagadananda Pandita says in Prema-vivarta:

gaura ye visala nama sei nama gau;
anya saba nama-mahatmya sei name pau.

"The most powerful names of God are the names of the most merciful incarnation of God, who is known as Lord Gauranga. The glories and power of all the other names of God are completely present in the Names of Lord Shri Gauranga Mahaprabhu and thus They should be constantly chanted."

(From: http://nitaai.com/nitaiveda/most_principal_name_of_god.html)