For sheer rhetorical power Martin Luther King's I Have a Dream speech is hard to beat, but perhaps his best polemical effort was his Letter from a Birmingham Jail. In terms of sheer moral force MLK's "Letter" is one of the great documents in American history.
On this day when we commemorate his life, It might be appropriate to urge our readers to invest some time reading his message, which he wrote to his fellow clergymen to explain why he, himself a clergyman, was engaged in civil disobedience. It will doubtless do more to help us understand this man than anything else we could do in ten or fifteen minutes.
A copy of the letter can be found here.
I'm sure you'll agree that it was worth your time to read.