Your test scores and grades may be good, but so are those of many other applicants. In fact, the average scores at many of the top institutions in the nation are remarkably high. Because of that, plus the fact that some colleges no longer even require standardized test scores, the admissions landscape has changed drastically for college applicants in the past decade. Today, college application essays have become the most influential component of the application process in many ways. Your college admissions essays are your best opportunity to communicate directly with the admissions officials, who look to college essays to find reasons to select one candidate over another. When you’re reviewing files from two candidates with equally impressive scores and grades, and you only have room for one, you have to use something to make your decision. That’s where essays come in.
Not just humor, but the overall tone of your application essay is remarkably important. It's also difficult to get right. When you are asked to write about your accomplishments, those 750 words on how great you are can make you sound like a braggart. Be careful to balance your pride in your achievements with humility and generosity towards others. You also want to avoid sounding like a whiner -- use your essay to show off your skills, not to explain the injustices that lead to your low math score or failure to graduate #1 in your class.
The popular "topic of your choice" option had been removed from the Common Application between 2013 and 2016, but it's now back again for the 2017-18 admissions cycle. Use this option if you have a story to share that doesn't quite fit into any of the options above. However, the first six topics are extremely broad with a lot of flexibility, so make sure your topic really can't be identified with one of them. 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. Cleverness is fine, but don't be clever at the expense of meaningful content.