Somewhat surprisingly to me, the University of Iowa. I'd heard great things from residents who'd interviewed there in previous cycles, but I was just blown away by this program. They have new facilities with art everywhere (e.g. huge hand painted derm textbooks from the early 1900s), tons of space (they even have a really nice room just for rotating medical students), their program director was awesome (more engaging/pleasant than any I've encountered on the trail so far), their faculty in general seemed really cool and approachable, their residents were great, they make all sorts of research opportunities available (basic science and clinical) without forcing it down resident's throats, it's a fairly big program, they have 1-2 half days of non-structured academic time per week (e.g. to go read, catch up on paperwork, etc.), 2 months elective time, etc. I also appreciated the fact that that they gave a really thorough and honest appraisal of their program. I've left some interviews not feeling like I had a good enough sense for the program as a whole, but that wasn't the case at all at Iowa.
I guess the only negative would be the location, but Iowa City actually seemed pretty cool -- sort of like a more low key version of Ann Arbor. I'm only about half way done interviewing so I can't say that I'll rank this place number 1, but I'll definitely rank it very highly, which I wouldn't have said prior to attending the interview.