OM KLEEM KRISHNAYA NAMAH (krishna mantra).... Also Hare Krishna, Hare Krishna, Krishna Krishna, Hare Hare. Hare Rama, Hare Rama, Rama Rama, Hare Hare.... (krishna mantra)....
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Shiva aka Rudra aka Tryambakam mantra is OM NAMAH SHIVAY (pronounce Shivaya)....
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Lord Vishnu mantra
OM NAMO NARAYANAYA.... Also OM NAMO VASUDEVAYA....
This mantras are everything.... However Gayatri Mantra the most Powerful mantra.... But if u are nonvegeterian than i dont recomment u Gayatri mantra. Infact Gayatri mantra was given to Vishwamitra (friend of everyone (whole world) after his the most powerful Penance (tapas).... This mantra was hidden by Brahmins due to fear that low caste people will get rich and get everything they want including wisdom.... Vishwamitra gave this mantra to humanity but Brahmins hidden it even when vishwamitra was not brahmin.... He hated brahmins and brahmins hated him until he became the most powerful saint.... Gayatri mantra changes human DNA within a week. 10 times reciting of gayatri mantra erase 1 day of sin.... 108 times erase whole month.... 1008 erase whole year sin. 10000 times erase whole life sin.... 1 million times reciting erase previous life and 10 million times reciting will give u Salvation and you will join heaven (its in other dimension).... i have tested all mantras.... To me above above mantras are powerful but Gayatri mantra and Mahamrityunjaya mantra are two most powerful mantra that will connect u directly with eternal which has no ending.... i have so much info but its sad people here totaly closed minded....