Being an American my views probably will be different than those of Brazilians and other nationalities, due to different histories and cultures. Therefore the readers of this post should consider those differences while considering the contents herein.
While the books of major religions such as the Bible, Torah, and Koran are rife with great and memorable speeches, I choose not to include them in my list of most memorable speechs as they are regarded as the words of God, and not of mere mortals.
My short list of most memorable speeches are: The Getteysburg Address by President Abraham Lincoln, The Quit India Speech by Mahatma Gandhi, The Ask Not What Your Country Can Do For You, Inaugural Address by President John F. Kennedy, and The I Have a Dream Speech by Dr. Martin Luther King. Each of these speeches are without a doubt memorable in their own right. However In my humble opinion "The I Have a Dream Speech" is the most memorable that I have read.
My reasons for this selection are: the way people of all races were motivated, the clear vision communicated to the nation and the world, the denouncement of violence, the long term relevence of the speech, and Dr. King's knowing refusal to bow to the dangerous political and social climate at the time.