When I was a kid, on Sundays my father would listen to a radio program called Evergreen, in which they played old songs that stay fresh forever. Unchained Melody is on of the few songs I explicitly remember, and it is an epitome of the style and feeling the show had. It has an eternal quality. Our grandparents danced to this song at their wedding, and our grandchildren will too.
Immediately when playing it you can recognize it as a classic. But curiously, the history of the song is filled with people not seeing this, and it is product of a series of coincidences. To start with, why is it called Unchained Melody? It is because it wasn't even originally released as a song. It was featured in a movie called Unchained, and thus the nameless song was called Unchained Melody. The guy who wrote the lyrics, initially said he couldn't because he was painting his house. There were some popular versions released following the movie in 1955. Then 10 years later, the Righteous Brothers took it up as a cover version. They flipped a coin about who was to sing it, and it was published as a B-side to some song no one knows today. Phil Spector who produced it, tried to force radio stations to play the A side instead. But even after all these roadblocks, the melody could not be kept chained. The song has 260 million listens on Spotify, which is far more than any other song from the Righteous Brothers (or indeed most artists).
There have been a large number of covers, which are just not as good. The Righteous Brothers had the vocal performance, and the larger-than-life production from Phil Spector. Markedly inferior versions by Robson and Jerome (1995) and Gareth Gates (2002) outsold it, but Spotify listeners have only given them 1-2 million listens. Elvis has the only other interesting version, released shortly before his death.
It is interesting to me how quality that seems obvious today, that even feels eternal, was not realized at the time. Time can do so much.