I believe that his music dramas are, by long odds, the most stupendous works of art ever contrived by man – that it took more downright genius to imagine them and fashion them than it took to build the Parthenon, or to write “Faust” or “Hamlet”, or to paint the Sistine frescoes, or even to write the Ninth Symphony.
H.L. Mencken 1921

Wagner, considered as an artistic force, was something almost without parallel, probably the most formidable talent in the entire history of art. Where else has there ever been such a conjunction of greatness and guile, of ingenuity and sublime depravity, of popularity and devilish finesse? He remains the paradigm of world-conquering artistry.
From a letter to an opera producer written by Thomas Mann 11/15/27

There is a musician who, more than any other musician, is a master at finding the tones in the realm of suffering, depressed, and tortured souls, at giving language even to mute misery. None can equal him in the colors of late fall, in the indescribably moving happiness of the last, truly last, truly shortest joy; he knows a sound for those quiet, disquieting midnights of the soul, where cause and effect seem to be out of joint and where at any moment something might originate ‘out of nothing.’ (…) …indeed, as the Orpheus of all secret misery he is greater than any.
From “Nietzsche contra Wagner” by Friedrich Nietzsche

When Verdi heard of Wagner’s death, he wrote to Giulio Ricordi of the powerful mark the composer would leave upon the history of art. But he reconsidered his observation; his pen cancelled “powerful” and in its place he wrote “most powerful” – “potentissima”.
From “Richard Wagner: The Man, His Mind, and His Music” by Robert Gutman