Milwaukee City Hall is my favorite, I believe its the only City Hall (perhaps even seat of government) to hold the title as World's Tallest Building (1895-1899).
Martin Saunders Flickr
It recently underwent a complete restoration including removing and repairing damaged masonry, re-tiling the roof, and completely replacing the copper on the bell tower. I find this renovation extremely fascinating because the building now looks brand new, its like going back in time and seeing the building as it first looked as a WTB.
During the renovation, you can see the new copper on the top of the bell tower. The whole building has been covered in scaffolding for nearly two years: