From what I read, Gaye's
What's Going On was one of the first real black protest records, or perhaps the first overtly political one from an African-American artist. Along with the likes of Gil Scott-Heron the album helped to give a voice to those who hadn't really had much say in things like Vietnam and racist crimes in America, and at its heart it has a really strong message of love and brotherhood across racial and faith lines. A great album that says, essentially, wake up everyone and look at what's happening.
And of course there's "Sexual Healing", so, you know...