If there is one thing in your life that you care about more than any other, talk about it in your essay. This works especially well if your passion requires that you attend a university. For instance, if your passion in life is protecting endangered sea creatures, and you want to become a marine biologist, write about how your education and academic background can help you reach those goals. Anything that is important to you and relevant to the school can work.
Attending college is a period of growth and change. Explaining how you have grown or changed over the last several years is one way to show an admissions board that you are ready for the challenges of a university. Focus on big things like values, interests and goals.
If you have faced a difficult situation in your life that strengthened you or changed who you are, discuss it in your essay. Talk about things that may have happened in your life and how you overcame those hardships. If you don't have any large events, try to think of ethical or moral dilemmas that were presented to you and how you dealt with them in the moment. Be sure to tie these issues back into your personal development.
Isolate what you believe to be your strongest personal trait and discuss why it will make you a successful student as well as a successful adult. Traits that work well for this topic include honesty, commitment, and positivity. Sorting out what makes you different from other applicants and how these differences make you the best choice is always good for your essay.
Whatever topic you choose, make sure your essay is thoroughly spell checked and proofread. Your topic should be accessible to the reader with vivid details of your personal experiences or opinions, but should not be based on anything that may offend a potential reader. If you are planning to take a chance on your essay with a gimmicky layout or controversial topic, be aware that not everyone will experience a positive reaction to that essay. Make your essay unique and noticeable -- you want admissions staff to remember it at the end of the day.