Prudential Center Skywalk

As second-tallest building in Boston, the somewhat-unloved Prudential Tower does have the redeeming quality of boasting the only publically-accessible sky-high observatory in town.  Unlike the observation deck at the flashier and slightly taller John Hancock Tower, the 50th floor Prudential Center Skywalk stayed open after the 9/11/2001 terrorist attacks in New York.  Nowadays, this 1965 vintage office tower provides the best view of historic and contemporary Boston through all 360 degrees.