In 1651 Thomas Hobbes used the term nosce teipsum which he translated as 'read thyself' in his famous work, The Leviathan . He was responding to a popular philosophy at the time that you can learn more by studying others than you can from reading books. He asserts that one learns more by studying oneself: particularly the feelings that influence our thoughts and motivate our actions. As Hobbes states, "but to teach us that for the similitude of the thoughts and passions of one man, to the thoughts and passions of another, whosoever looketh into himself and considereth what he doth when he does think, opine, reason, hope, fear, etc., and upon what grounds; he shall thereby read and know what are the thoughts and passions of all other men upon the like occasions." 
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.
Avoid slipping into clichés or generalities. Take this opportunity to really examine an experience that taught you something you didn't previously know about yourself, got you out of your comfort zone, or forced you to grow. Sometimes it's better to write about something that was hard for you because you learned something than it is to write about something that was easy for you because you think it sounds admirable. As with all essay questions, the most important thing is to tell a great story: how you discovered this activity, what drew you to it, and what it's shown you about yourself.