The medium in which you convey a message can be quite important to the quality of the message that you want to make public. It can influence how persuasive the message is, how much people listen to it, and how powerful it proves.
In this case, King and Malcolm X both use speeches, while Angelou uses a poem. A speech is generally more persuasive because it allows the listener to hear the voice of the person, and with it, his personal tone and intonation. Moreover, speeches are generally more effective at creating a specific call to action. On the other hand, Angelou's poem is not as effective when it comes to action. However, it can still be very powerful because of the author's use of imagery and literary language.