"If there were no virtues of Shaykh Taqi ad-Deen except for his famous student Shaykh Shams ad-Deen ibn al-Qayyim al-Jawziyyah, writer of many works, from which both his opponents and supporters benefited from then this would be a sufficient indication of his (ibn Taymiyyah’s) great position. And how could it be otherwise when the Shaafi`ee Imaams and others, not to speak of the Hanbalees, of his time testified to his prominence in the (Islamic) sciences…" Ibn Katheer said,
[From Ibn Hajr’s endorsement of ‘Radd al-Waafir’ contained at the end of the book.]
"the least he would do when he heard something was to memorise it and then busy himself with learning it. He was intelligent and had much committed to memory, and he became an Imaam in tafseer and everything linked to it and knowledgeable in fiqh. Indeed it was said he was more knowledgeable of the fiqh of the madhabs then the followers of those very same madhabs in his time and other than his time. He was a scholar in Usul and the branches of the religion and grammar and the language and other textual and intellectual sciences…no scholar of a science would speak to him except that he thought that that science was the speciality of ibn Taymiyyah. As for hadeeth then he was the carrier of its flag, a haafidh and able to distinguish the weak from the strong, fully acquainted with the narrators…"
[‘al-Bidaayah wan Nihaayah’ (14/118-119) of ibn Katheer.]