The five-syllable mantra in honor of Shiva – Namah Shivaya.
It is the Mahamantra – great mantra – of the Shiva devotees.
Vishnu is honored with eight syllable mantra – Ashtakshara – Om namo Narayanaya.
The Mahamantra of the Krishna devotees is a 32-syllable mantra
Hare Krishna Hare Krishna Krishna Krishna Hare Hare Hare Rama Hare Rama, Rama Rama Hare Hare.