In terms of looking in from the outside, ANY derm program would be considered very difficult to get into, and thus a "top program". Looking from the inside, it's very much a question of "chemistry" - Can you get along with the PD/attendings? How is the workload? Do you aim for a career in academic derm or in private practise? Do you want to focus on melanomas or the more cosmetic procedures?
I have my own favorites, but it's a fairly personal thing, so I'm not sure it'll be very helpful to anybody else...