This is about as subjective as it comes, but there are several contenders, mostly relatively ancient but from the same decade: Abbey Road (Beatles), Bridge Over Troubled Water (Simon and Garfunkel), Deja Vu (CSN&Y), Dark Side of the Moon (Pink Floyd), Court and Spark (Joni Mitchell), Desperado (Eagles), Goodbye Yellow Brick Road (Elton John), Chicago VII (Chicago), The Stranger (Billy Joel), 52nd Street (Billy Joel), and Hotel California (Eagles). Also, In the Digital Mood— Glenn Miller music, released in 1991.
Since I have to pick one, I’ll go with Abbey Road.