Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The greatest blessing Allah has bestowed upon His servants is the blessing of faith and guidance to Islam, knowing Allah, worshipping Him alone, and following His Messenger ﷺ. Allah says:
“Today I have perfected your religion for you, completed My favor upon you, and chosen Islam as your religion.”
Guidance to the religion is the greatest blessing because it is the means of attaining Allah’s pleasure and Paradise.
Is It Correct to Say, “The Greatest Blessing Is That Allah Is Our Lord”?
This statement can carry a correct meaning: that Allah has honored the believer by enabling him to know Him and believe in Him. However, it is more precise to say:
The greatest blessing Allah has bestowed upon us is that He guided us to believe in Him, worship Him alone, and follow His religion.
Allah, Glorified and Exalted, is the Lord of all creation, believers and disbelievers alike. However, this Lordship becomes a complete blessing for the one who believes in Him and follows His guidance.
Are Worldly Blessings the Greatest Blessings?
Wealth, health, children, and other worldly blessings are great blessings, but they become complete blessings when they are used in obedience to Allah. The blessing of faith, however, is the lasting blessing that benefits a person in this world and the Hereafter.
Conclusion
The greatest blessing Allah has bestowed upon His servants is guidance to faith, worshipping Him alone, and following His Messenger ﷺ. It is permissible to praise Allah for being our Lord, but it is more precise to say: The greatest blessing is that Allah has guided us to faith and Islam. This is the blessing through which a person attains true happiness in this world and the Hereafter.
And Allah knows best.