What is the top of the building called?

ANS1:The ball at the top of the pole is called a "Truck". Snopes concurs (see link below) If you call it anything but a truck you were not in the military or you slept through boot camp. We called them trucks in the Navy and in the Army and I presume the Marines and Air Force called them trucks, too.

The running joke back in the day to ask a recruit (before everybody started calling them trucks) was "How many trucks are on an Army post?" Answer: One--on top of the flagpole. Those other things are called "Vehicles, wheeled"...now drop and give me 20.

ANS2:The ball or decorative top on a flag pole is called a final, The truck is the bolt that holds the final to the pole.

(If you want to call it a "finial" or "final", as if it were just a decorative cap on a fence post, you could...but anybody who has been in the military will take the time to correct you.)