Shaykh Yasir Qadhi delves into the topic of the Greatest or Grandest Name of Allah (Ism-Allah Al-Adham).
There are a few authentic Hadith which mention the Greatest Names of Allah:
The Prophet ﷺ saw somebody make Dua and said "You have asked Allah with His Grand Name, this is the Name if asked using it, Allah will grant and Dua is answered."
A person made Dua and the Prophet ﷺ said "This man has invoked Allah's Grand Name, when it is used, Allah responds and when Dua is made, Allah gives"
The Prophet ﷺ said: "The grand name with which when Dua is made,it is granted is found in 3 Surahs - Baqarah, Ale Imran and Taha"
The Prophet ﷺ said the Grand Names were Ar-Rahman, Ar-Raheem and Hayyul Qayyum.
It is beneficial to say all of the Names at once to ensure nothing is skipped by saying - "Al-Ahad, As-Samad, Al-Mannan, Al-Bari, Dhul Jalali Wal Ikram, Al-Hayy Al-Qayyum.