There is a lot of (secret) mantras, some of which have never ever been available to the public. In the video below, however, there are four mantras introduced which carry the supreme essence of all other mantras, in my opinion.
What is the use of the 3 Om mantra, or Vajrasattva mantra, or Heart
The Buddhist Master, Thich Nhat Hanh:
“Dear friends, dear people, I know that you suffer a lot. I have not understood enough of your difficulties and suffering. It is not our intention to make you suffer more. It is the opposite. So please, tell us about your suffering, your difficulties. I am eager to learn, to understand.”