I think three easiest and most powerful at the same time are:
"Aham Prema" (I am divine Love) With this simple mantra, you affirm your own essential nature as One of Divine Love. The vibration of this mantra begins the process of transforming your entire body, being, nature, and spirit into emanation, your own particular emanation, of Divine Love.
"Om Namah Shivayah" It means "I bow to Shiva." Shiva is the supreme reality, the inner Self. It is the name given to consciousness that dwells in all. Shiva is the name of your true identity- your self. A very rough, non-literal translation could be also be something like, 'Om and salutations to that which I am capable of becoming.'
"Om Shanti Om" It means Peace : Literally creates the sound vibration of peace and invokes peace of mind.
The best thing to do is to concentrate on one mantra at a time for 40 days until it becomes a part of you
This is the Ganesh Mantra, you will receive all the luck, blessings in the world. Its really powerful I've used it before and continue to use it. Imagine all your worries, obstacles and path being cleared to your dreams. Enjoy! xx
Its great that I came across your comment. I recently discovered this mantra and I'm chanting it randomly during the day and sometimes I set a timer and chant for 5 min. I really want to know more about its efficacy and how to best use it. I am really wanting to manifest a job at a particular firm and I am facing some bumps on the road. Can you tell me how I can use this mantra better to clear my path? What is the best way to chant it? I read about doing this for 5 min every day for 40 days at a stretch. Do you know about it? Please tell me more. Thanks.