Task: Question Answering
Input (English): Passage: Solti recorded throughout his career for the Decca Record Company. He made more than 250 recordings, including 45 complete opera sets. During the 1950s and 1960s Decca had an alliance with RCA Victor, and some of Solti's recordings were first issued on the RCA label.Solti was one of the first conductors who came to international fame as a recording artist before being widely known in the concert hall or opera house. Gordon Parry, the Decca engineer who worked with Solti and Culshaw on the Ring recordings, observed, "Many people have said 'Oh well, of course John Culshaw made Solti.' This is not true. He gave him the opportunity to show what he could do."Solti's first recordings were as a piano accompanist, playing at sessions in Zurich for the violinist Georg Kulenkampff in 1947. Decca's senior producer, Victor Olof did not much admire Solti as a conductor (nor did Walter Legge, Olof's opposite number at EMI's Columbia Records), but Olof's younger colleague and successor, Culshaw, held Solti in high regard. As Culshaw, and later James Walker, produced his recordings, Solti's career as a recording artist flourished from the mid-1950s. Among the orchestras with whom Solti recorded were the Berlin Philharmonic, Chicago Symphony, London Philharmonic, London Symphony and Vienna Philharmonic orchestras. Soloists in his operatic recordings included Birgit Nilsson, Joan Sutherland, Régine Crespin, Plácido Domingo, Gottlob Frick, Carlo Bergonzi, Kiri Te Kanawa and José van Dam. In concerto recordings, Solti conducted for, among others, András Schiff, Julius Katchen, Clifford Curzon, Vladimir Ashkenazy and Kyung-wha Chung.Solti's most celebrated recording was Wagner's Der Ring des Nibelungen made in Vienna, produced by Culshaw, between 1958 and 1965. It has twice been voted the greatest recording ever made, the first poll being among readers of Gramophone magazine in 1999, and the second of professional music critics in 2011, for the BBC's Music Magazine. 
Question: What Solti recording  has twice been voted the greatest recording ever made?
Output (English): Wagner's Der Ring des Nibelungen.