Audrey Hepburn (as Holly Golightly) singing a song in one of my favourite movies: Beakfast at Tiffany's.
Although the president of Paramount Pictures didn't like it and actually wanted it removed from the film, the song became a great succes and even won Henry Mancini an Academy Award in 1962.
The original soundtrackalbum of the movie doesn't contain the version of Moon River sung by Audrey. Instead it contains a version by Mancini and his chorus, which was released as a single and became a hit. Personally I prefer Audrey:
Audrey Hepburn - Moon River