The John Hancock Tower is the tallest building in New England, however, the 60th floor observatory that used to be open to the public has been closed since 9/11. It's still worth going, as this I.M. Pei designed building is still awe-inspiring, even from ground level. When you're close to the building, you get some great reflections of the nearby historical buildings, and are steps away from Copley Square and Trinity Church.
To see if there are any changes in the status of the observatory, call 1-617-572-6425. I hope that they re-open soon!