Can't give an easy answer to this. As someone who sleeps and breathes music, the list would go on for ever, but here are a few favourites.
Classical - anything and everything by Claude Debussy. The Bruch Violin Concerto and "The Flower Duet" from Lakme by Delibes.
Pop - Everything by David Gates and 'Bread'. Simon and Garfunkle, James Taylor, Janis Ian, Leonard Cohen, Pink Floyd, Led Zeppelin, The Beatles. The Carpenters. 10cc. Queen. David Bowie.
Musicals - Les Miserables. Ragtime. Chicago. Fiddler on the Roof. West Side Story. Rent.