That is a tough one to define, you'd have to develop some kind of 'weighting' formula to come up with a coherent list.
OTOH, I am always kind of impressed with the Mayo Clinic in Rochester, MN. I'm not sure how tall it is, but Rochester is a very smallish city (2000 USCensus population - 92,507) with pretty much all of its metro area inside of the city limits.
That Northern Illinois University building in Freeport, IL (2000 USCensus population - 25,867) is quite disproportionate, too.
Mike