If you want to be an engaging speaker, your interest is paramount.
Yes, you want to draw a crowd but if the audience cares more about your talk than you do yourself, it's a recipe for failure.
Ideally, you choose the topic
You're speaking at a conference and get to pick what you'll talk about.
To choose a good topic, ask yourself:
What would you like to write a book about?
What's an exciting project you worked on recently?
What have you talked to your co-worker/spouse/friends about and they could tell how excited you were?
How to work with a topic someone picked for you
You don't always get to choose your topic.
When someone else decides for you, make the topic exciting for you (if it currently isn't).
For example, if your manager asks you to present the department results at the next internal 'all hands' meeting and you're not wild about the idea, try the following:
Are there interesting reasons why the numbers are above or below target?
How would YOU address the necessary performance improvements?
Could you add a different type of Q&A session at the end of your presentation?
Could you highlight the work of your colleagues?
Your excitement is what makes a talk engaging
An enthusiastic presenter can read the phone book and be entertaining.
Find a way to make public speaking fun for yourself first and foremost. You'll send out that energy to your audience and they'll hang on your words, no matter the topic.