Best place to start is What's Going On. A true classic soul album. Beautifully sequenced and arranged, powerful lyrics and godly vocals. For many it's the definitive soul album.
After that you can't go wrong with Let's Get It On, I Want You and Hear, My Dear.
Hear, My Dear is my personal favorite. A judge made him record the album and give the proceeds of the sales to his ex-wife. Marv decided to record an angry album covering the whole divorce. It's probably the GOAT break up album. It's an ugly, nasty record but I love it for how honest it is.