By any measure of speeches or leadership, Martin Luther King’s “Why I Am Opposed to the War in Vietnam” must rank highly, no matter what your thoughts on the politics of the situation.
The text (text here) is eloquent, erudite, colorful, and informative. His delivery was rousing, engaging, inspiring, and on par with any other of his speeches.

(The link in case the video isn’t showing — https://www.youtube.com/watch?v=FfGsVvnvA9w)
As effective as this speech was, and not that they were competing, but Muhammad Ali has to get the nod on eloquence and effectiveness.

No Vietcong ever called me nigger.

Whereas King’s speech covers nearly every issue, at 4,468 words, few would listen to his speech twice or pass much of its message to others. It inspires, but its content doesn’t engage nearly like Ali’s.
Meanwhile, Ali’s both inspires and gets you thinking. It’s hard to say, after reading his quote, “Well, you should still go fight them for people who do call you nigger.” It gets you asking who his friends and enemies are — who your friends and enemies are.
I should learn to say so much so succinctly and poignantly.