Imaam Abdur Rahmaan bin Mahdi rahimahullah (D. 198 H) said:

“This (Ahmed bin Hanbal) was the most knowledgeable person among the people concerning the hadeeth of (my teacher) AthThawree”
[Al-Jarh wat Ta’deel: 1/292; 2/68, Chain Saheeh]

Imaam Muhammad bin Muslim bin Waarah ar-Raazi rahimahullah
(D. 270H) said concerning Imaam Ahmed:

“He is a man of Fiqh, a man of (Strong) Memory, and a man of understanding”
[Al-Jarh wat Ta’deel: 1/294, Chain Saheeh]

Imaam Abu Zur’ah Ubaydullah bin Abdul Kareem ar-Raazi
rahimahullah (D. 264 H) said:

“I do not know any one among our companions, whose hair color is black (i.e. he is young), to be more Faqeeh than Ahmed bin Hanbal”
[Al-Jarh wat Ta’deel: 1/294, Chain Saheeh]

Imaam Abu Zur’ah said:

“I have not seen anyone more Jaami (all-encompassing) than Ahmed bin Hanbal, and neither a more complete person than him, (the qualities like) asceticism, excellence, Fiqh, and a lot more things have gathered in him”
[Al-Jarh wat Ta’deel: 1/294, Chain Saheeh]

Imaam Ali ibn Abdullaah al-Madeeni rahimahullah (D. 234H) said:

“There is none, among our companions, a bigger a Haafidh than Abu Abdullah Ahmed bin Hanbal, and it has reached me that he does not narrate except through a Book, and in it is good example for us.”
[Al-Jarh wat Ta’deel: 1/295; 2/69, Chain Saheeh]

Imaam Ibn al-Madeeni said:

“Ahmed bin Hanbal is our leader”
[Hilyat al-Awliyaa: 9/165, 171, Chain Saheeh; Taareekh Baghdaad: 4/417, Chain Saheeh; Manaaqib Ahmed P. 109, Chain Saheeh]

Amr bin Muhammad bin Bukayr al-Naaqid rahimahullah (D. 232H) said:

If Ahmed bin Hanbal accords me in (the narration of) a hadeeth, then I do not care who opposes me.
[Al-Jarh wat Ta’deel: 1/296, Chain Hasan]

Muhaddith Kabeer Imaam Ahmed bin Sinaan al-Waasiti rahimahullah (D. 259H) said:

“I have never seen Yazeed bin Haaroon praising anyone more than Ahmed bin Hanbal. He used to make him (Ahmed) sit beside him when he would narrate a hadeeth to us; and when Ahmed got sick, Yazeed bin Haaroon rode to him to inquire his well-being.”
[Al-Jarh wat Ta’deel: 1/297, Chain Saheeh]

Imaam Yazeed bin Haaroon used to respect Imaam Ahmed to a great extent. [See, Al-Jarh wat Ta’deel: 1/297, Chain Saheeh; Manaaqib Ahmed: P. 68, Chain Saheeh]

Taken from the works of Shaykh Zybayr Ali Zai, translated by Raza Hassan, the book can be downloaded from here