book tower in detroit was the first building that immediately came to my mind too, but i don't know if it is in fact the tallest exposed fire escape in the world.
if it ain't THE tallest, it at least has to be up near the top. there are only a handful of cities we could even consider because there aren't many 500+ foot pre-war buildings in the world (book tower is 475'). and i can't think of the 500+ footers in pittsburgh, cincy, cleveland, columbus, baltimore or chicago having a taller exposed fire escpae than book tower, so that would mean only NYC might have a potentially taller one.
book tower
source:
http://www.skyscrapercity.com/showth...=248304&page=3