Write about something meaningful and describe your feelings, not necessarily your actions. If you do this, your essay will be unique. Many people travel to foreign countries or win competitions, but your feelings during these events are unique to you. Unless a philosophy or societal problem has interested you intensely for years, stay away from grand themes that you have little personal experience with. For some reason, students continue to think big words make good essays.
Big words are fine, but only if they are used in the appropriate contexts with complex styles.
Use Imagery and Clear, Vivid Prose. If you are not adept with imagery, you can write an excellent essay without it, but it's not easy. The application essay lends itself to imagery since the entire essay requires your experiences as supporting details. Appeal to the five senses of the admissions officers. Spend the Most Time on your Introduction.