The Greatest Name of Allah
Question

It was narrated from the Prophet (blessings and peace of Allah be upon him) that he said: “Allah has ninety-nine names; whoever memorises them will enter Paradise,” and one of these names is the greatest name of Allah which, if He is called upon thereby, He will answer and if He is asked thereby, He will give. 
My question is: what is the greatest name of Allah which if He is called upon by it, He will answer and if He is asked by it, He will give? How can we know it? Did any of the scholars know it? Are the scholars unanimously agreed upon it? May Allah reward you with good.
Praise be to Allah.Reference to the greatest name of Allah in the Sunnah
A number of Hadiths were narrated concerning the “greatest name of Allah ,” the most famous of which are the following:

It was narrated from Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “The greatest name of Allah is in three Surahs of the Quran: Al-Baqarah, Al `Imran and Ta-Ha .” (Narrated by Ibn Majah, 3856; classed as sound by Al-Albani in Sahih Ibn Majah)
It was narrated from Anas (may Allah be pleased with him) that he was sitting with the Messenger of Allah (blessings and peace of Allah be upon him), and a man was praying then he said in supplication: O Allah, I ask You by virtue of the fact that all praise is due to You; there is none worthy of worship but You alone, and You have no partner or associate, the Bestower, the Originator of the heavens and the earth, the Possessor of majesty and honour, O Ever-Living, O Sustainer. The Prophet (blessings and peace of Allah be upon him) said: “He has asked Allah by His greatest name which, if He is called upon thereby He answers and if He is asked thereby He gives.” (Narrated by At-Tirmidhi, 3544, Abu Dawud, 1495, An-Nasa’i, 1300 and Ibn Majah, 3858. Classed as authentic by Al-Albani in Sahih Abu Dawud.)
It was narrated from Buraydah ibn Al-Husayb (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) heard a man saying: O Allah, I ask You by virtue of the fact that I bear witness that You are Allah, there is none is worthy of worship but You, the One, the Eternal Refuge, Who neither begets nor is born, nor is there to Him any equivalent. The Messenger of Allah (blessings and peace of Allah be upon him) said: “He has indeed asked Allah by His greatest name, which if He is asked thereby, He gives and if He is called upon thereby, He answers.” (Narrated by At-Tirmidhi, 3475, Abu Dawud, 1493, Ibn Majah, 3857; classed as authentic by Al-Albani in Sahih Abu Dawud)

Ibn Hajar (may Allah have mercy on him) said:
This is the soundest of all reports concerning this matter in terms of its Isnad (chain of narration).

It was narrated from Asma’ bint Yazid (may Allah be pleased with her) that the Prophet (blessings and peace of Allah be upon him) said: “The greatest name of Allah is in these two verses: `And your god is one God. There is no deity [worthy of worship] except Him, the Entirely Merciful, the Especially Merciful’ [Al-Baqarah 2:163] and at the beginning of Surah Al `Imran: `Alif, Lam, Mim. Allah - there is no deity except Him, the Ever-Living, the Sustainer of existence’ [Al `Imran 3:1-2].” (Narrated by At-Tirmidhi, 3478, Abu Dawud, 1496 and Ibn Majah, 3855)

This Hadith is inauthentic. Its Isnad includes `Ubaydullah ibn Abu Ziyad and Shahr ibn Hawshab, both of whom are weak narrators.
Scholars’ views on the greatest name of Allah
The scholars differed concerning the “greatest name of Allah” and whether there is such name. There are several views:

The first view is that there is no such name! Because they believe that none of the names of Allah, may He be Exalted, can be regarded as superior to another. These scholars interpreted the Hadiths quoted above in various ways:


Some said that what is meant by the word Al-A`tham (the greatest) is Al-`Athim (the great), and that there can be no differentiation between the names of Allah, may He be Exalted.

Ibn Hajar (may Allah have mercy on him) said:
“Some scholars – such as Abu Ja`far At-Tabari, Abu Al-Hasan Al-Ash`ari and a number of others after them, such as Abu Hatim ibn Hibban and Al-Qadi Abu Bakr Al-Baqillani, rejected that and said that it is not permissible to regard some of the divine names as being superior to others. Some of them attributed that view to Malik, because he regarded it as disliked to habitually recite or repeat one Surah rather than others, lest it be thought that some parts of the Quran are better than others, and that lead to believing that the less favoured parts are inferior to those that are more favoured. They interpreted the reports concerning that as indicating that what is meant by “the greatest” is “great”, and that all the names of Allah are great. When Abu Ja`far At-Tabari said: The reports differ as to which is the greatest name, and in my view all opinions are sound, because none of these reports state that it is the greatest name, and nothing is greater than Him, it was as if he was saying: each of the names of Allah, may He be Exalted, may be described as the greatest, in the sense that they are all great, as noted above.”

That what is intended in the Hadiths quoted above is to highlight the extra reward for the one who calls upon Allah by that name.

Ibn Hajar (may Allah have mercy on him) said:
“Ibn Hibban said: What is meant by the greatness mentioned in the reports is the extra reward granted to the one who calls upon Allah by that name, just as there are Hadiths which speak of great reward for the one who recites the Quran; what is meant thereby is the extra reward of the reciter.” 

That what is meant by the greatest name is the frame of mind and focus of the one who is calling upon Allah by that name, which includes everyone who calls upon Allah, may He be Exalted, by any of His names, if he has the right frame of mind.

Ibn Hajar (may Allah have mercy on him) said:
“It was said that what is meant by the greatest name is any of the names of Allah, may He be Exalted, by which a person calls upon Him with complete submission and focus of mind, in the sense that at that moment he has nothing in his mind except Allah, may He be Exalted. Whoever is like that will receive a response. Something similar to this was narrated from Ja`far As-Sa diq, from Al-Junayd, and from others.” 

The second view is the view of those who say that Allah, may He be Exalted, has kept to Himself knowledge of His greatest name, and that He has not granted knowledge thereof to any of His creation.

Ibn Hajar (may Allah have mercy on him) said:
“Others said: Allah, may He be Exalted, has kept to Himself knowledge of His greatest name, and that He has not granted knowledge thereof to any of His creation.” (See: Fath Al-Bari, by Ibn Hajar, 11/224)

The third view is the view of those who affirmed that the greatest name of Allah exists and that it is a specific name, but they differed as to which is the greatest name, and there are fourteen views! Ibn Hajar (may Allah have mercy on him) has listed them in his book Fath Al-Bari (11/224-225). They are as follows:


Hu! (He!)
Allah
Allah Ar-Rahman Ar-Rahim (Allah the Most Gracious, the Most Merciful)
Ar-Rahman Ar-Rahim Al-Hayy Al-Qayyum (the Most Gracious, the Most Merciful, the Ever-Living, the Sustainer)
Al-Hayy Al-Qayyum (the Ever-Living, the Sustainer)
Al-Hannan Al-Mannan Badi` As-Samawati wa’l-Ard Dhul-Jalali wal-Ikram Al-Hayy Al-Qayyum (the Compassionate, the Bestower, the Originator of the heavens and the earth, the Possessor of majesty and honour, Ever-Living, Sustainer)
Badi` As-Samawati wa’l-Ard Dhul-Jalali wal-Ikram (the Originator of the heavens and the earth, the Possessor of majesty and honour)
Dhul-Jalali wal-Ikram (the Possessor of majesty and honour)
Allahu la ilaha illa Huwa Al-Ahad As-Samad alladhi lam yalid wa lam yulad wa lam yakun lahu kufuwan ahad (Allah there is none worthy of worship but He, the One, the Eternal Refuge, who neither begets nor is born, Nor is there to Him any equivalent)
Rabb Rabb (Lord, Lord)
The prayer of Dhun-Nun in the belly of the fish: la ilaha illa anta subhanaka inni kuntu min Ath-thalimin (There is none worthy of worship  except You; Exalted are You. Indeed, I have been of the wrongdoers) [Al-Anbiya’ 21:87 -  interpretation of the meaning]
Huwa Allah Allah Allah alladhi la  ilaha illa Huwa Rabb Al-`arsh Al-`athim (He is Allah, Allah, Allah, besides Whom there is no other god, Lord of the mighty Throne)
It is concealed within the divine names
The word of Tawhid: La ilaha ill-Allah (there is none worthy of worship but Allah).

Shaykh Al-Albani (may Allah have mercy on him) said:
“It should be understood that the scholars differed as to which is the greatest name of Allah, and there are fourteen views, which were listed by Ibn Hajar in Al-Fath, followed by the evidence for each view. Most of the evidence is from the Hadiths, but in some cases it is simply the view of an individual and no attention need be paid to it, such as the twelfth view, the evidence for which is that so and so asked Allah to teach him the greatest name, then he saw in a dream that it was “Allah Allah Allah alladhi la  ila ha illa Huwa Rabb Al-`arsh Al-`athim (He is Allah, Allah, Allah, besides Whom there is no other god, Lord of the Mighty Throne)”!
Some of these Hadiths are authentic, but are ambiguous in meaning, and others are Mawquf [a statement of a Companion], like this one. Some of them are clear in meaning; they fall into two categories:
The first category is that which is sound and clear in meaning, namely the Hadith of Buraydah (may Allah be pleased with him), “Allah, there is none worthy of worship  but He, the One, the Eternal Refuge, Who neither begets nor is born…” Ibn Hajar (may Allah have mercy on him) said: This is more sound in terms of its isnad than everything else that has been narrated concerning that. And it is as he said (may Allah have mercy on him). Ash-Shawkani confirmed that in Tuhfat Adh-Dhakirin (p. 52). It is listed in Sahih Abu Dawud (1341).
The second category is those which are clear in meaning but are not sound, some of which Ibn Hajar (may Allah have mercy on him) clearly stated were inauthentic, such as the Hadith which is quoted to support the third view, which was narrated from `Aishah (may Allah be pleased with her) in Ibn Majah (3859). It also appears in Da`if Ibn Majah (841). Concerning some other reports he did not say anything, such as the Hadith which is quoted to support the eighth view, namely the Hadith of Mu`adh ibn Jabal (may Allah be pleased with him) in At-Tirmidhi, which is listed in Ad-Da`ifah (no. 4520).
There are other Hadiths that are clear in meaning, but Ibn Hajar (may Allah have mercy on him did not quote them; however, they are Wahin (flimsy); they are also listed in the same book (nos. 2772, 2773 and 2775). (Silsilat Al-AHadith Ad-Da`ifah wa’l-Mawdu`ah, 13/279)
The  correct view regarding the greatest name of Allah
Perhaps the view that is most likely of all of them to be correct is the view that the greatest name is “Allah,” because this is the one name which encompasses all the other names and attributes of Allah, may He be Exalted, and it is a name that was never given to anyone other than Allah, may He be Exalted. This is the view of most scholars.

Ibn Al-Qayyim (may Allah have mercy on him) said:

“The name “Allah” is indicative of all the beautiful divine names and sublime attributes…” (Madarij As-Salikin (1/32)

Ibn Amir Haj Al-Hanafi (may Allah have mercy on him) said:

“It was narrated that Muhammad ibn Al-Hasan said: I heard Abu Hanifah (may Allah have mercy on him) say: The greatest name of Allah is “Allah”. This is also the view of At-Tahawi and many of the scholars, and most of the devoted worshippers.” (At-Taqrir wat-Tahbir (1/5)

Abu Al-Baqa’ Al-Fatuhi Al-Hanbali (may Allah have mercy on him) said:

“We learn two things:

The first is that the name “Allah” is a name that refers to His essence and is unique to Him, so it encompasses all of His beautiful names.
The second is that it is the greatest name of Allah according to most of the scholars, and is the name that is connected to all praiseworthy attributes.” (Sharh Al-Kawkab Al-Munir, p. 4)


Ash-Shirbini Ash-Shafi`i (may Allah have mercy on him) said:

According to some scholars, it is the greatest name of Allah. It is mentioned in the Glorious Quran in two thousand three hundred and sixty (2360) places.” (Mughni Al-Muhtaj ila Ma`rifat Alfaz Al-Minha j (1/88, 89)

Shaykh `Umar Al-Ashqar (may Allah have mercy on him) said:

“What appears to be the case from comparing the texts in which the greatest name of Allah is mentioned is that this name is “Allah”. This name is the only name which appears in all the texts concerning which the Messenger (blessings and peace of Allah be upon him) stated that the greatest name of Allah is mentioned in them.
One of the things which support the opinion that “Allah” is the greatest name is the fact that it is repeated in the Holy Quran 2697 times – according to the list in Al-Mu`jam Al-Mufahras [a famous concordance or index of the Quran] – and it is mentioned in the form “Allahumma” [a vocative form] five times, whereas another divine name which is applicable only to Allah, may He be Exalted – namely Ar-Rahman [the Most Gracious] – is mentioned only fifty-seven times. This idea is also supported by the fact that this name (“Allah”) encompasses many great and significant meanings.” (Al-`Aqidah fi Allah (Belief in Allah), p. 213)
The second of these views in terms of strength is the view that the greatest name of Allah is Al-Hayy Al-Qayyum (the Ever-Living, the Sustainer). This is the view of a number of scholars, including An-Nawawi (may Allah have mercy on him), and it was regarded as more likely to be correct by Shaykh Al-`Uthaymin (may Allah have mercy on him).
For more details, please see the following answers: 11278 , 260383 , and 246242 . 
And Allah knows best.