Swami Vivekananda considered Adi Shankaracharya (Adi Shankara) as the greatest teacher of Vedanta philosophy. Vivekananda was an admirer of Adi Shankara and translated Adi Shankara's poem Atma Shatakam (also known as Nirvana Shatakam). Here is the Wikipedia article on Atma Shatakam and here is Swami Vivekananda's translation of the poem.
In this post, we'll make a collection of Swami Vivekananda's quotes and comments on Adi Shankaracharya or Adi Shankara.
Swami Vivekananda told—
Did India ever stand in want of reformers? Do you read the history of India? Who was Ramanuja? Who was Shankara? Who was Nânak? Who was Chaitanya? Who was Kabir? Who was Dâdu? Who were all these great preachers, one following the other, a galaxy of stars of the first magnitude? Did not Ramanuja feel for the lower classes? Did he not try all his life to admit even the Pariah to his community? Did he not try to admit even Mohammedans to his own fold?
... his whole life’s work is nothing but that, the throbbing of the beauty of the Vedas and the Upanishads.
I do not know why Shankara should be represented as rather exclusive; I do not find anything in his writings which is exclusive. As in the case of the declarations of Lord Buddha, this exclusiveness that has been attributed to Shankara's teachings is most possibly not due to his teachings, but to the incapacity of his disciples.
Shankara came, a great philosopher, and showed that the real essence of Buddhism and that of the Vedanta are not very different, but that the disciples did not understand the Master and have degraded themselves, denied the existence of the soul and of God, and have become atheists. That was what Shankara showed, and all the Buddhists began to come back to the old religion.
Shankara says, Brahman is the essence, the reality of all knowledge, and that all manifestations as knower, knowing, and known are mere imaginings in Brahman.