Ranking residency programs is a very difficult (nearly impossible) and probably fruitless task. The reasons for this have been alluded to by other posters:
- Many many many factors go into a medical student's choice of residency. Prestige is only one. Others include location, the people there, academic vs community, research opportunities, conferences, etc.
- Looking at the quality of the hospital/department may be of some help, but you have to be careful. Just because a deparment receives high marks in US News, does not mean its training program is stellar. These rankings are based on patient care, not resident education, and these two are not necessarily correlated.
Residency really is a "program that is best for you" rather than "program that is best" decision. Or at least it ought to be.
Realistically, I think you will receive a fine education at either NYU or Northwestern. You should just decide which school you liked best and whether you want to be in NYC or Chicago. They will both have fine residents and attendings and will educate you well.
As for the question regarding ob/gyn programs...I know nothing about these. However, I would say that you are probably best off going to a school that has decent training in all fields, not just one. The reason for this is that you cannot really know what specialty is best for you until you try things out. Most med students who go into medical school with an idea regarding specialty end up changing their minds. You want good exposure to a variety of fields.