Meaning of “Allah is The Greatest”
25-11-2018 | IslamWeb
Question:
Salamu Aleikum. What does "Allahu akbar" mean exactly? Does it means Allah (SWT) is great or Allah (SWT) is greater? If "greater" than in comparison to who?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
We have already explained the meaning of the phrase "Allah is The Greatest" in the Arabic Section in Fatwa 49147. We have mentioned therein that the meaning of "Allah is The Greatest", i.e. greater than everything.
Since you write to us sometimes in Arabic, then please refer to this Fatwa.
Here, we will add the statement of Imam ash-Shaatibi 
in clarifying the superiority between the qualities of the Creator and the characteristics of the creature, although there is no proportion of comparison between them, as he said:
"The superiority in regard to Allah is in comparison to the custom of the creatures in addressing each other, and according to their normal imagination (or understanding); His Saying "Allah is The Greatest", means greater than anything that could be imagined to be great, or according to what they used to distinguish between the creatures, although the Greatness of Allah does not proportion to the greatness of a creature; such as His saying: {…and that is [even] easier for Him.} [Quran 30:27]; He means according to your usual custom, that the reinvention of whatever has been invented before is easier than inventing it at the first place. His saying: {He was most knowing of you.} [Quran 53:32]; i.e. than you, as you presume that you have knowledge, and that Allah has knowledge, or according to what you say: “This one is more knowledgeable than this one”. This is the way of the Arabs in their speech, and the Quran was descended with it [language of the Arabs]; so they were addressed according to their speech [language], and what they were accustomed to among themselves." [End of quote]
Allah knows best.