I think the individual makes the doctor, but rank in a small way helps the person choose the location that they want to practice. That being said, I agree with Jefe that I'm a little dissapointed in the residency directors rank. I think that might have to do with the quality of teaching that you get there. Also I'm guessing that even though you get a lot of experience in County, they don't have much time to actually do the teaching.
Temple is in a similar situation that USC is in in that they have great clinical, but they are understaffed so the medical students get tons of experience. he told me that residency directors tell him that Temple students are better than other students the first few months of residency because they know their way around more and know more methods. But the problem is that they didn't have time to read papers and study during Temple's 3rd and 4th years, so they are not as cutting edge as the other students and eventually the other students catch up and pass the Temples students because of this.
So that is one theory about why USC could be ranked low by the residency directors.