Remember that while selecting the greatest enlightening topic for you, you must consider your audience, your hobbies, as well as your time and length constraints. Then, adapt the fundamental concept to your own scenario. For example, if you are invited to give a speech on "Environmental Protection", you should discuss issues related to the environment and how they affect people today. You can also include topics such as climate change, deforestation, pollution etc. In addition, select a topic that you are interested in. This will make you more enthusiastic and effective when giving the speech. Finally, remember that your topic selection is only the beginning. The rest is up to you! You need to expand on it by creating interesting content and delivering it effectively if you want your audience to remember you after your speech.