Mariah Carey's "All I Want for Christmas is You" sells the equivalent of 2 million copies each year, if not more, and the song is at 14x platinum now. This upcoming year is the 30th anniversary of the 1994 Merry Christmas album she put out, and she has an accompanying 20 city tour to support the promo. With all of that being said, I believe in just a few short years "All I Want for Christmas is You" will be the biggest selling song of all time.