I'll go with Bob Dylan and Bruce Cockburn.

As I said in that other thread, most of what Dylan writes contains strong elements of protest, as he constantly questions the status quo, rails against the phoniness of the entertainment business and society in general, and points out in no uncertain terms the decadence that is all around us.

Buffy Sainte-Marie is also an extraordinary protest singer.

And Eric Bogle is pretty notable as well.