http://admissions.yale.edu/advice-putting-together-your-application#essay
Students commonly want to know what part of the college application “carries the most weight.” The truth is, there are many parts to your application, and together they help us discover and appreciate your particular mix of qualities. Academic criteria are important to Yale’s selective admissions process, but we look at far more than test scores and grades.
Every applicant brings something unique to the admissions committee table. Perhaps one application stands out because of sparkling recommendations, while another presents outstanding extracurricular talent; maybe your personality shines through a powerful written voice, or maybe your keen mathematical mind packs more punch. Our goal is to assemble a diverse, well-rounded freshman class, and that means admitting exceptional individuals of all types. You may find this answer unsatisfying, but we assure you that it is true: the part of the application that carries the most weight is different from applicant to applicant.
This section of our website aims to help you submit the very best application possible. We asked admissions officers to weigh in with their own thoughts on each topic and we have compiled their responses below. We know that the application process can be confusing, daunting, even overwhelming, and we hope this page proves helpful as you compile your applications, not only to Yale but to every school on your list.