Overall, you need to do something you enjoy. You also need to do your thesis on a topic that relates to what you plan on doing with your degree at some point. If you don't, you risk making yourself known for a topic that you don't care about, or you risk being bored and struggling to finish your thesis within the allocated time. Your advisor and thesis committee will help you make the difficult decisions that you need to make and will help steer you toward topics you will excel at. Don't try to do it on your own, it will make it more difficult for you in the end.