In his book, Introduction to the Madhhab of Imam Ahmad b. Hanbal, Shaykh Abdul al-Qadir b. Badran dedicated an entire chapter highlighting the reasoning behind many of the great scholars choosing the school of Imam Ahmad over other schools.
He mentions that Ibn al-Jawzi, said, "know that we have examined the legal evidence, principles of jurisprudence, and probed into the lives of the luminary mujtahids and found this man - Imam Ahmad - to be the most knowledgeable."
"He was from those who preserved the book of Allah and recited it to the authorities of his time."
"As for hadith, everyone has agreed that he stands out over all others due to the sheer number of narrations he has reported, his knowledge of the authentic from the weak, and mastery of hadith sciences."
"It has been affirmed that there is no one to have preceded him in the mastery of narrations like that of Imam Malik and if someone wanted to know the position of Imam Ahmad in comparison to Imam Malik, he or she is to look no further than the difference between al-Musnad and al-Muwatta'."
(al-Musnad by Imam Ahmad has roughly 40,000 narrations whereas al-Muwatta' by Imam Malik has roughly 3000 narrations).
"Abu Na'eem al-Hafiz reported the following discourse from Imam al-Shafi'i;
'Muhammad ibn al-Hasan (one the foremost disciples of Imam Abu Hanifah and student of Imam Malik) asked him, 'Is our Imam (i.e. Abu Hanifa) more knowledgeable or yours (i.e. Imam Malik)?'
Imam al-Shafi'i: 'Do you want to stubbornly quarrel or be fair?'
Muhammad ibn al-Hasan: 'Be fair.'
Imam al-Shafi'i: 'What religious proof do you use?'
Muhammad ibn al-Hasan: 'The Qur'an, Sunnah, consensus, and qiyas.'
Imam al-Shafi'i: 'By Allah, is our Imam more knowledgeable of Allah's book or yours?'
Muhammad ibn al-Hasan: 'If you are asking me by Allah, then yours.'
Imam al-Shafi'i: 'Is our Imam more knowledgeable of the Prophet's sunnah or yours?'
Muhammad ibn al-Hasan: 'Yours.'
Imam al-Shafi'i: 'Is our Imam more knowledgeable of the companions' narrations or yours?'
Muhammad ibn al-Hasan: 'Yours.'
Imam al-Shafi'i: 'Is there anything remaining other than qiyas?'
Muhammad ibn al-Hasan: 'No.'
Imam al-Shafi'i: 'Well, we claim qiyas more than you do. Qiyas is known on how it is made based on the fundamentals.'"
Ibn al-Jawzi went on to say, "Imam al-Shafi'i's discourse regarding Imam Abu Hanifa is sufficient. We have also come to know the degree of our Imam (i.e. Ahmad ibn Hanbal) for he achieved what Malik achieved and much more which has been mentioned regarding the comparison between al-Musnad and al-Muwatta.
Imam al-Shafi'i was a scholar of many sciences; he did, however, submit that Imam Ahmad was more knowledgeable in the field of narrations which represents the axis of fiqh.
Imam Ahmad told his son, Abdallah that Imam al-Shafi'i told him, 'you are more knowledgeable than me in Hadith. If a hadith is authentic, let me know so I can follow it.'"
"Unlike the other Imams, he could mention the issues of al-jarh wa al-tadil from memory just like reciting al-Fatihah when asked. Anyone who has examined the book al-Ilal by Abu Bakr al-Khallal knows this."
"Without argument, he was also unmatched in his knowledge of the verdicts, rulings, consensus, and the differing opinions of the companions."
"As for Arabic, it has been reported that he said, "I have written more about Arabic than Abu Amr al-Shaybani".
"As for Qiyas, there is simply too much to mention regarding his deductive reasoning."
"Ahmad combined what he possessed of knowledge with what others were unable to of zuhd/asceticism and scrupulousness. It has not been mentioned that any of the scholars refused the stipend of the rulers and gifts of others like him."
"He stood alone patiently with the Mihna/Trial defending the truth."
Abu al-Qasim al-Jabbuli said that "many people think that Imam Ahmad is largely remembered for the Mihna/Trial which is not true. If Imam Ahmad b. Hanbal was asked about something, it was as if all the knowledge of the world was at his fingertips."
For these reasons and more ibn al-Jawzi, "stood by the decision to choose the school of Imam Ahmad".
If the Hanbali School is so great, why are there so few followers?
Imam Abu al-Wafa Ali ibn Aqil al-Baghdadi said, "The adherents of this school have not done it justice. If the adherents of Abu Hanifa and al-Shafi'i excelled in the religious sciences they would be appointed as judges or other official offices.
Their appointments became the reason for them to teach and continue to pursue knowledge. As for the Hanbali disciples, when they learned the slightest bit, they would be motivated to immerse themselves in worship and asceticism due to their superior level of righteousness and would cease their pursuit of knowledge."