Class rank is “very important” for Yale admissions:
http://www.collegedata.com/cs/data/college/college_pg02_tmpl.jhtml?schoolId=244
However, that is far from the only criterion for admission. An applicant needs to be top-end in many other things as well (top-end test scores, extracurricular achievement at the national level, excellent recommendations, excellent essays, no “defects”) to have much of a chance of admission to a super-selective school.