Until someone does a better study, this is the only list that im aware of which determined their rank list based on objective measures. Whether you agree or disagree with their choice of variables is another point entirely!
http://dermatology.cdlib.org/133/original/academy/table6.htm
Obviously, the strength of clinical training is of utmost importance for someone deciding their residency program. It's much harder to grade this objectively, but a great training program should have a strong general, surgical and dermpath group to teach their residents. Secondly, a wide variety of patients is important. Lastly, a good program should have a strong track record of getting their residents placed into good fellowships/jobs. All other things are personal preference IMHO.