What type of speech does the professor want? What are the requirements? What are the time restrictions for the presentation?
Who will hear the speech? All topics are not made equally. A speech about Quantum Physics would not be appropriate for a class of kindergartners.
Pick a topic that you would like to hear about. Chances are, if you enjoy speaking about your topic, your audience will enjoy listening to it. Also, when you are more interested in a subject, it is easier to learn and teach others about it.