What is the best prayer to offer God?
Lets hear it from Krishna(God) himself what is the best boon to ask from him.
This is an event narrated in Srimad Bhagwatam. The Devtas were defeated by the great demon Vratasura and they offer prayers to Vishnu for protection.
Prayers by the devtas to please Vishnu
The Supreme Personality of Godhead, who created us by His external potency and by whose mercy we expand the creation of the universe, is always situated before us as the Supersoul, but we cannot see His form. We are unable to see Him because all of us think that we are separate and independent gods.
The demigods said: O Supreme Personality of Godhead, You are competent to give the results of sacrifice, and You are also the time factor that destroys all such results in due course. You are the one who releases the chakra to kill the demons. O Lord, who possesses many varieties of names, we offer our respectful obeisances unto You.
Therefore, O Lord, O supreme controller, O Lord Kṛṣṇa, please annihilate this dangerous demon Vṛtrāsura, Tvaṣṭā’s son, who has already swallowed all our weapons, our paraphernalia for fighting, and our strength and influence
Thus the devtas glorify Vishnu and then ask for a way to defeat Vratsura.
Vishnu is pleased but displeased
Lord Vishnu the Supreme Personality of Godhead appears in front of the devtas and addresses them as follows:-
SB 6.9.47 [1]— The Supreme Personality of Godhead said: O beloved demigods, you have offered your prayers to Me with great knowledge, and I am certainly most pleased with you. A person is liberated by such knowledge, and thus he remembers My exalted position, which is above the conditions of material life. Such a devotee is fully purified by offering prayers in full knowledge. This is the source of devotional service to Me.> SB 6.9.48 —** O best of the intelligent demigods, although it is true that nothing is difficult for one to obtain when I am pleased with him, a pure devotee, whose mind is exclusively fixed upon Me, does not ask Me for anything but the opportunity to engage in devotional service.**
SB 6.9.49 — Those who think material assets to be everything or to be the ultimate goal of life are called misers [kṛpaṇas]. They do not know the ultimate necessity of the soul. Moreover, if one awards that which is desired by such fools, he must also be considered foolish.
SB 6.9.50 — A pure devotee who is fully accomplished in the science of devotional service will never instruct a foolish person to engage in fruitive activities for material enjoyment, not to speak of helping him in such activities. Such a devotee is like an experienced physician, who never encourages a patient to eat food injurious to his health, even if the patient desires it.**
Thus from this story we come to know that Krishna or God is very pleased if we ask him pure devotional service. Those who ask material things from God are wasting their opportunity and are acting like misers. Krishna or God is anyways the ultimate protector and maintainer of all living entities. If we misuse the great opportunity by asking for material things we are not very intelligent.