Aretha Franklin - Amazing Grace (specifically the complete recordings if you can find it)

Aretha Franklin - I've Never Loved A Man The Way I Love You

Sam Cooke - Live at the Harlem Square Club '63

Marvin Gaye - What's Going On?

Stevie Wonder - Songs in the Key Of Life

A lot of the best R&B was not really written around the LP format, so definitely get some "Best of" compilations of Smokey & The Miracles, The Crystals, The Supremes, The Four Tops and that kind of classic Motown stuff too, but those five albums above are all absolutely essential.