PauranicMantra
Om Namo Bhagavate Vasudevaya
The twelve-syllable Vishnu mantra that opens the Bhagavata Purana.
Bhagavata Purana 1.1.1Vishnu12 syllables
PauranicMala mantra (long)
Hare Krishna Mahamantra
The sixteen-name mantra, given in the Kali-Santarana Upanishad.
Kali-Santarana Upanishad 1Krishna32 syllables