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.
There is a real disconnect between what Admission Committee Representatives in the Info Sessions say they are looking for and these essay topics. College Reps say they would like to know something interesting about you that isn’t revealed in your application. Your essay is supposed to add “texture” to your application. These new essay topics will produce really boring essays, with many students beating their own chests or writing sad tales about a death in the family or another type of loss. Bring back the topic of choice!