IMO don't worry too much on ranking. Instead, looking for high quality faculty members that interest you the most. Some schools with high ranking might not match your field best, some might even mostly because of their large faculty. Instead, schools with relative low rankings can have really good PIs. Like Boston U and Boston College, they ranked kind of low, and it's mostly because of the small faculty (some might not even cover all the fields in chemistry? I don't know), yet they are really strong for organic. Anyway, just make sure that you eventually pick up a school where you can find a professor to spend 5+ yrs with passion. Good luck :D