I doubt that rankings exist for a field that's as competitive as derm, but I know from my fiancee (who will be a PGY-1 next year) that the following programs are among the top nationally...
University of Pennsylvania - oldest department, excellent training
Harvard Medical School - well-balanced program with excellence in medical care and research
Yale Medical School - research and clinical training very good
New York University - largest dermatology program, many fine faculty members
University of Michigan - good research and clinical training
UCSF- good research and clinical experience
U Texas Southwestern- well balanced clinical and research activities
Stanford - new chairman has stabilized the department
To answer your follow-up questions YES it is extremely competitive and YES you will need top scores to match successfully (my fiancee has score >250 on Step I, and attends a top-10 school, which has offered her a position outside the match). Many programs assure their own students spots outside the match (technically illegal, but done), so fewer spots are available than are listed. If you don't already have an 'in' at a program, you'd better be the top of the top.