Agree with intparent. But how do you know you are one of the top in the country? Is there any verification of this? If so, you want to include that info.
You get one shot at displaying what really makes you tick. You do this in the order you list your activities/ECs and how you describe them.
But you also need to temper this with this question: does this make me attractive to colleges? What does my activity/skill/EC add to a future college class?
Your cube solving ability is probably of 4th or 5th order of importance. Really. It adds almost nothing to what an admissions officer is wanting in her class.
Present your features that make you attractive as a future collegian. Not something that would tickle gramma during Thanksgiving dinner
I had a few hobbies like this that I added as additional activities in the Common App section. It seems like it can definitely add to/differentiate your app from others, I would just stray away from making it the focus. YMMV.
Honestly, this is probably something that your guidance counselor should mention in his/her letter of recommendation as a quirky thing, but if I were you, I would fill up your application with other, more “college-like” (for lack of a better word) activities. Regarding extracurriculars, colleges want athletes, researches, dancers, artists––anyone that can HELP THEIR PROGRAMS––they don’t need a Rubik’s cube solver.
Colleges aren’t REALLY expecting freshman to bring something amazing that changes the whole school – they want interested and interesting people. This could be an aspect of being interesting, I think. Your essay is your chance to talk about how you have approached it, what opportunities it might have opened up for you, maybe a chance at some self-deprecating humor (but have a responsible adult read it over to make sure it is appropriate and funny to an adult reader, students aren’t always the best judge of this). A great essay can turn on something that isn’t dramatic or ‘adult’.