The early favorite seems to be "What's Going On?" by Marvin Gaye and I can't say I disagree...
I mean:
Off the top of my head, I can think of "This is America" by Childish Gambino as the only "hit" protest song in recent memory. Maybe Kendrick "Alright" counts.
This is a hip-hop head reacting to hearing Rage Against the Machine for the first time. Wait for it....It does not disappoint.
Breathtaking:
Handful of Senators don't pass legislation
Marches alone can't bring integration
Human respect is disintegratin
You tell me over & over & over again my friend
You don't believe we're on the eve of destruction