Ain't No Mountain High Enough — Marvin Gaye and Tammi Terrell (1967)

Written by Nickolas Ashford and Valerie Simpson in a Harlem kitchen in 1966 and first recorded by Marvin Gaye and Tammi Terrell for Motown, this song's call-and-response structure between two voices created the template for the soul duet. When Tammi Terrell collapsed in Gaye's arms onstage in October 1967 from what was later revealed to be a brain tumor, their partnership ended -- but their recordings together remain the emotional pinnacle of the Motown era.