Om Mani Padme Hung

That is the most famous mantra, of Avalokiteshvara. It mean Jewel-Lotus, a symbol of the Bodhisattva of compassion. Constant repetition is said to bring the uneducated, who do not practice formal meditation, to enlightenment.

As a recommendation, search on youtube for Imee Ooi. Her chants are beautiful and very powerful.