The first step to writing a stellar personal essay on your college application is to understand your options. Below is a discussion of the six essay options from the Common Application.
Note the key word here: evaluate. You aren't just describing something; the best essays will explore the complexity of the issue. When you examine the "impact on you you need to show the depth of your critical thinking abilities. Also, don't equate "topic of your choice" with a license to write a comedy routine or poem (you can submit such things via the "Additional Info" option). Essays written for this prompt still need to have substance and tell your reader something about you.
Introspection, self-awareness and self-analysis are all important here. And be careful with essays about the winning touchdown or tie-breaking goal. These sometimes have an off-putting "look how great I am" tone and very little self-evaluation. Option #4. Describe a character in fiction, a historical figure, or a creative work (as in art, music, science, etc.) that has had an influence on you, and explain that influence.
How do you contribute? Option #6. Topic of your choice. Sometimes you have a story to share that doesn't quite fit into any of the options above. However, the first five topics are broad with a lot of flexibility, so make sure your topic really can't be identified with one of them. Your readers have probably seen a lot of essays talking about what a great role model Mom or Dad or Sis is. Also realize that the "influence" of this person doesn't need to be positive.