NYC is so much bigger and denser than anywhere else. I think it's pretty easy to pick out. SF, if you include a bridge and the TransAmerica building is pretty hard to miss. Chicago is--I think--a little less obvious, but still easy with the Sears (is it still Sears? Probably not) and the Hancock Towers. Seattle, with the Space Needle, is pretty clear. LA, with the round, record-shaped (Columbia?) building is straightforward. Washington, D.C. is impossible not to spot. Of those, I would think LA is the least known.