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					Originally posted by  Negev
					 
				 
				
			
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 The Story of "The Greatest Thing":
(with information summarized from Gagapedia)
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					Originally posted by Gagapedia (abridged)
					
				 
				"The Greatest Thing" is a song written in 2007 by Lady Gaga and RedOne.  In 2011, Cher announced that Gaga had given the song to her for her upcoming studio album.  Although Cher was originally the only person on the track, Gaga and Cher both expressed an interest in doing it as a duet, and had RedOne produce it.  The duet was supposed to be released in September of 2011, but was delayed in production.  Gaga mentioned in 2012 that she disliked her vocals on the track.  In the summer of 2013, Cher answered a fan, and said that she loved Gaga's vocals and her own on the track, but RedOne's contract dictated that he be the only one allowed to touch the production, and he basically abandoned the song due to its "old" production. 
			
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 So, basically, the song got stuck in production hell by Gaga at first due to hesitancy about her vocals on the track, and now by RedOne because he doesn't want to work on it any more.  Apparently, there IS a completed version, and 17 people have it according to Cher, but it hasn't been released or leaked, only the demo version.
Here's the full article: 
http://ladygaga.wikia.com/wiki/The_Greatest_Thing_(song)
It contains references to a few interviews, and has links to various tweets made by Gaga and Cher.