There are two mantras called Mahamantra – great/exalted Mantra.

The one of the Shiva devotees
Namah Shivaya – Glory to God
It is called Panchakshara, five-syllable (mantra)

The one of the Vishnu/Krishna/Rama (all three are one) devotees
Hare Krishna Hare Krishna Krishna Krishna Hare Hare
Hare Rama Hare Rama Rama Rama Hare Hare.