Haafidh Ibn Hibbaan said:

“He (Imaam Ahmed bin Hanbal) was Haafidh, excellent, Pious, Faqeeh. He strictly abode by observing secret religiousness and Daaimi worship. Allaah helped the Ummah of Muhammad (sallallaahu alayhi wasallam) through him. This is (true) by (the fact) that he remained firm in times of trial, and devoted himself to Allaah, and got ready to be martyred. He was beaten by stones. Allaah saved him from Kufr, and made him a person worth following. He was such a shelter that people used to find shelter under him.” [Ath-Thiqaat of Ibn Hibbaan: Vol. 8 P. 18/ 19]

Imaam Ahmed bin Abdullah bin Saalih al-Ijlee rahimahullah (D. 261 H) said:

He (Imaam Ahmed) is (Thiqah) Thabat in Hadeeth; He is the possessor of a clean soul, and a Faqeeh in Hadeeth. He is an adherer, who adheres to the Athaar (Traditions). He was a man of Sunnah (Sunni) and pious.
[Ath-Thiqaat of Al-Ijlee: 9; Al-Taareekh al-Baghdaad: 4/415, Chain Saheeh; and the addition of Thiqah in brackets is from that]

Imaam Abu Haatim Muhammad bin Idrees ar-Raazi rahimahullah (D. 277 H) said:

“When you see a man loving Ahmed bin Hanbal, then know that this man is a man of Sunnah”
[Al-Jarh wat Ta’deel: 1/308, Chain Saheeh]

“He is an Imaam, and he is a Hujjah (evidence – in Deen)”
[Al-Jarh wat Ta’deel: 2/70]

“Ahmed bin Hanbal had a very good understanding of the Authentic and Weak narrations”
[Al-Jarh wat Ta’deel: 1/302, Chain Saheeh]

Imaam Abu Haatim has declared Ahmed bin Hanbal to be a bigger Faqeeh than Ali ibn al-Madeeni
[Al-Jarh wat Ta’deel: 1/294, Chain Saheeh]

Imaam Abu Raja Qutaybah bin Sa’eed ath-Thaqafi rahimahullah (D. 240H) said:

“Ahmed bin Hanbal is the Imaam of whole world.”
[AL-Jarh wat Ta’deel: 1/295; 2/69, Chain Saheeh]

“Ahmed bin Hanbal and Ishaaq bin Rahwayh are Imaams of the whole world”
[Taareekh Baghdaad: 4/417, Chain Saheeh]

“If Ahmed bin Hanbal had witnessed the era of Ath-Thawree, Maalik, Al-Awzaa’ee, and Layth bin Sa’d then he would have been a superior (over them).”
(Abdullah bin Ahmed bin Shabwayh said) I asked Qutaybah: Is Ahmed bin Hanbal compared to the Taabi’een? He replied: “(Yes, he is compared) to the Major Taabi’een”.
[Al-Jarh wat Ta’deel: 1/293; 2/69, Chain Saheeh]

Imaam Qutaybah has preferred Imaam Ahmed over Imaam Yahya bin Yahya and Imaam Ishaaq bin Rahwayh.
[Al-Jarh wat Ta’deel: 1/293; 2/69, Chain Saheeh]

“When you see a man loving Ahmed bin Hanbal, then know that he is a man of Sunnah and Jamaa’ah”
[Al-Jarh wat Ta’deel: 1/308, Chain Saheeh]

“Had there been no Ahmed bin Hanbal, religious zeal would have died”
[Hilyat al-Awliyaa: 9/168, Chain Saheeh]

“Had Ath-Thawree not been there, righteousness would have died; and had Ahmed not been there, people would have inserted Bid’aat (Innovations) in the religion”
[Taareekh Baghdaad: 4/417, Chain Saheeh]

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