Short answer
The Prophet ﷺ spoke of Allah's greatest name (al-ism al-a'zam): when Allah is called by it, He answers. He did not single it out plainly, and scholars differ. Many say it is the name "Allah"; others say "al-Hayy al-Qayyum". Hadith point to Ayat al-Kursi, the opening of Al Imran and a dua with "al-Ahad as-Samad".
There is no binding ruling on which name it is; it is a matter of virtue. Many Hanafi scholars held it is the name Allah, since every other name is attributed to it. Calling on Allah by His names is mustahabb and a sunnah of dua.
The four schools agree on this.
Explanation
Several sound hadith speak of the greatest name. The Prophet ﷺ heard a man make dua: "O Allah, I ask You, as I bear witness that You are Allah, there is no god but You, the One (al-Ahad), the Self-Sufficient (as-Samad), who neither begets nor is born, and there is none equal to Him." He said the man had asked Allah by the name which, when He is asked by it, He gives, and when He is called by it, He answers (Abu Dawud, graded sahih).
In another hadith he said the greatest name is in two verses: "And your god is one God. There is no deity except Him, the Entirely Merciful, the Especially Merciful" (2:163) and the opening of Al Imran, "Alif Lam Mim. Allah, there is no deity except Him, the Ever-Living, the Sustainer" (Tirmidhi, graded hasan). Other reports point to al-Hayy al-Qayyum, which also appears in Ayat al-Kursi (2:255).
From these, scholars drew different conclusions. Many said the name is "Allah" itself, since it appears in all of these duas. Others said "al-Hayy al-Qayyum", or "al-Ahad as-Samad". Some said it was hidden on purpose, like Laylat al-Qadr, so that people call on Allah by all His names.
The practical lesson: begin your dua with praise of Allah, use His names that fit your need, and include "Allah, la ilaha illa Huwa, al-Hayy al-Qayyum". Allah says, "To Allah belong the best names, so invoke Him by them" (7:180). Learn them on our 99 Names of Allah page, with al-Hayy and al-Qayyum.
The evidence
Quran, Surah Al-Baqara 2:163
وَإِلَٰهُكُمْ إِلَٰهٌۭ وَٰحِدٌۭ ۖ لَّآ إِلَٰهَ إِلَّا هُوَ ٱلرَّحْمَٰنُ ٱلرَّحِيمُ ﴿١٦٣﴾
And your god is one God. There is no deity [worthy of worship] except Him, the Entirely Merciful, the Especially Merciful.
One of the verses the hadith says contains the greatest name.
Quran, Surah Al-Baqara 2:255
ٱللَّهُ لَآ إِلَٰهَ إِلَّا هُوَ ٱلْحَىُّ ٱلْقَيُّومُ ۚ لَا تَأْخُذُهُۥ سِنَةٌۭ وَلَا نَوْمٌۭ ۚ لَّهُۥ مَا فِى ٱلسَّمَٰوَٰتِ وَمَا فِى ٱلْأَرْضِ ۗ مَن ذَا ٱلَّذِى يَشْفَعُ عِندَهُۥٓ إِلَّا بِإِذْنِهِۦ ۚ يَعْلَمُ مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ ۖ وَلَا يُحِيطُونَ بِشَىْءٍۢ مِّنْ عِلْمِهِۦٓ إِلَّا بِمَا شَآءَ ۚ وَسِعَ كُرْسِيُّهُ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضَ ۖ وَلَا يَـُٔودُهُۥ حِفْظُهُمَا ۚ وَهُوَ ٱلْعَلِىُّ ٱلْعَظِيمُ ﴿٢٥٥﴾
Allah - there is no deity except Him, the Ever-Living, the Sustainer of [all] existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and whatever is on the earth. Who is it that can intercede with Him except by His permission? He knows what is [presently] before them and what will be after them, and they encompass not a thing of His knowledge except for what He wills. His Kursi extends over the heavens and the earth, and their preservation tires Him not. And He is the Most High, the Most Great.
Ayat al-Kursi contains al-Hayy al-Qayyum.
Quran, Surah Al-A'raaf 7:180
وَلِلَّهِ ٱلْأَسْمَآءُ ٱلْحُسْنَىٰ فَٱدْعُوهُ بِهَا ۖ وَذَرُوا۟ ٱلَّذِينَ يُلْحِدُونَ فِىٓ أَسْمَٰٓئِهِۦ ۚ سَيُجْزَوْنَ مَا كَانُوا۟ يَعْمَلُونَ ﴿١٨٠﴾
And to Allah belong the best names, so invoke Him by them. And leave [the company of] those who practice deviation concerning His names. They will be recompensed for what they have been doing.
Commands calling on Allah by His names.
Sunan Abi Dawud 1493
Grading to be checked... لقد سألت الله بالاسم الذي إذا سئل به أعطى وإذا دعي به أجاب ...
"You have asked Allah by the name which, when He is asked by it, He gives, and when He is called by it, He answers."
Confirms a greatest name exists and points to the dua with al-Ahad as-Samad.
Jami' at-Tirmidhi 3478
Grading to be checked... اسم الله الأعظم في هاتين الآيتين ...
"Allah's greatest name is in these two verses."
Points to 2:163 and the opening of Al Imran.
Where the ruling is recorded
- Mirqat al-Mafatih (Mulla Ali al-Qari), Kitab al-Da'awat, chapter on the names of Allah. A Hanafi commentary that sets out the views on which name is the greatest.
- Sharh al-Aqa'id al-Nasafiyya (al-Taftazani), Section on the names of Allah. Explains that Allah's names are taken from revelation.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.