Michael Jackson had the best-selling album two years in a row with "Thriller." BSR Agency/Gentle Look via Getty Images Billboard, using data recorded by Nielsen Soundscan/Luminate, tracks each year's best-selling album. Since 2015, streaming has been incorporated into music metrics through the use of album-equivalent units. Many classic albums don't appear on the list, having been outsold by lesser-known records. Each year, one album taps into the hearts and ears of the public on its way to becoming the year's best-selling album. Times have changed, though. In recent years, singers like Adele and Taylor Swift have dominated the charts, while in the 1950s and early 1960s, musical soundtracks and Broadway cast recordings often topped the list.  Billboard has been tracking the best-selling album of the year since 1956, but its methodology has changed several times.…