Saturday Night Fever, The Bee Gees + Various Artists, 1977. Interestingly, while Saturday Night fever has a pumpin' soundtrack, it's a pretty grim, urban film. Aesthetically the cover is revolting but it sold a bundle.
http://pongogirl2.hubpages.com/hub/1970s-Fashion - Bee Gees...I believe they had the top five song positions on the charts. It got to the point that they wrote songs for other singers. John Travolta sealed the whole disco look. I personally loved the disco era.