Om is the most powerful chant to the Hindu.

The symbol Om written in Sanskrit represents everything. The material world of the
waking state is symbolized by the large lower curve. The deep sleep state is
represented by the upper left curve. The dream state, lying between the waking state
below and the deep sleep state above, emanates from the confluence of the two. The point and semicircle
are separate from the rest and rule the whole. The point represents the state of
absolute consciousness. The open semicircle is symbolic of the infinite and the fact
that the meaning of the point can not be grasped if one limits oneself to finite thinking.

The chanting of Om drives away all worldly thoughts and removes distraction and infuses
new vigour in the body. The chanting of Om is a powerful tonic. I believe when chanted correctly
it is simply breathing in and out, breathing in during the O and out during the M.
Repeated often it simply clears your lungs and makes you feel better.

Here is the most famous mantra... The Gayatri Mantra

OM BHOOR BHUWAH SWAHA,
TAT SAVITUR VARENYAM
BHARGO DEVASAYA DHEEMAHI
DHIYO YO NAHA PRACHODAYAT.

Translation = Oh God! Thou art the Giver of Life, Remover of pain and sorrow,
The Bestower of happiness, Oh! Creator of the Universe, May we receive thy supreme
sin-destroying light, May Thou guide our intellect in the right direction.