I would vote for STL becuase of the vacant space there, formerly used by TWA, and the fact that the most populated 2/3 of the country is a short flight or drive away. Of course, the eastern part of the US is also home to some fierce airline competition.

Does anyone remember Western Pacific Airlines? They had a hub in COS, during their brief existence. With DIA such a long drive from some parts of Denver, it actually made sense to fly out of COS sometimes. Now that the suburban sprawl on the front range has nearly completely linked all of the cities, it might be time to take a look at that idea again.