Which Mantra To Use?
Firstly there are several types of mantras.
- Single word
- Multiple words
- Sentences (or Verse)
- Poetic (+ Many Sentences)
Single word
The most common example of a single word mantra is the word OM.
However, you can use any single word you choose.
Multiple words
Possibly the 2 most commonly known Vedic mantras - in the West may be.
- Om Mani Padme Hum
- Om Nama Shiva Ya
- Om Tare Tu Tare Ture So-ha
Om Mani Padme Hum
IMO, this is the most well known 4 words in the world of meditation.
"Om Mani Padme Hum"
is well known as a feel good mantra.
Om Nama Shiva Ya
Possibly the number 1 mantra contained in Rudram is the Panchakshara Mantra, "Om Nama Shiva-ya". 
It is important to note that many Vedic words have a "soft" or even silent "A" or "H" sound at the end of the word.
Some people pronounce the "A" at the end and some people do not.
For example: Buddha is commonly referred to as The Buddh and the Mahabharata is called the Mahabharaht.
While many in the West say this mantra with a YAH at the end, I have also heard this without the accented "A" as well,
"like this video"
.
It should be noted that the "object" she is singing to is a Shiva Linga (Lingam) and no matter how wikipedia and many websites like to put a silk dress on the meaning, the fact that it looks phallic and sits within a "bowl" shaped like a woman's private parts should explain how 99,999% of people see this "object".
I am telling you this so you have full disclosure.
BTW: I love this woman's name.
Om Tare Tu Tare Ture So-ha
This emotionally moving rendition of Om Tare Tu Tare Ture So-ha is better known as "The Green Tara Mantra"
is here BUT the "supposed" translation is PURE Propaganda.
Sentences or Stanzas
A "Stanza" is a group of lines forming the basic recurring metrical unit typically found in a poem.
While there are Vedic mantras utilizing both sentences and stanzas, I would like to switch gears for a moment to biblical references.
I am doing this because these next 2 examples completely sum up what I have learned in the last 49 years of research.
It may surprise many people, but the Dalai Lama also preaches this exact same message as these 2 sentences.
- Psalm 27:4
- Deuteronomy 6:5
Psalm 27:4
- One thing I ask from the Lord, one thing I desire.
- That I might dwell in Your house all the days of my life.
- To behold the graciousness of the Lord, and to enter God's sanctuary.
More info on "Psalm 27:4" 
Deuteronomy 6:5
And I love the lord my god with all my heart, all my soul and all my essence.
More info on "Deuteronomy 6:5". 
Mrytunjaya Mantra
Rudram also have a long chain mantra which is the great "Mrytunjaya Mantra"
.
This is not recommended for Christians. See translation below.
Transliteration
- Aum Tryambakam yajaamahe sugandhim pushtivardhanam.
- Urvaarukamiva bandhanaan-mrityormuksheeya maamritaat.
Translation
- We worship the three-eyed One (Shiva), who is fragrant and who nourishes all.
- Like the fruit falls off from the bondage of the stem, may we be liberated from death, from mortality.
It is always important to know what the mantra means before you start saying it.
Poetic
Apart from biblical verses which many people may be aware of, there is some Vedic prose which may be used.
Nirvana Shatakam
The absolute number 1 among these is The "Nirvana Shatakam"
by the great sage "Adi Shankara". 
It describes what God is Not.
This is not a good mantra for arrogant people - or perhaps it is the perfect mantra.
Hidden within the Nirvana Shatakam mantra is an even more powerful mantra called the "Mano Buddhi Mantra". 
This is the "Mano Buddhi Study"
showing extremely heightened Gama Oscillation Waves.
His famous Nirvana Shatakam is an enigma hidden within a question where the answer comes before the question and then repeats with another question and answer - but you cannot see any of them until they are pointed out. I explain one example here on the "Adi Shankara Riddle"
page.