What is the best floor of a building to throw a cat from?

Any floor above the seventh!

Higher than the seventh floor, it doesnt really matter how far the cat falls, as long as its oxygen holds out.

Like many small animals, cats have a nonfatal terminal velocity--in cats this is about 60 mph. Once they relax, they orient themselves, spread out, and parachute to the ground like a squirrel.

There are cats on record that have fallen thirty stories or more without ill effects. One cat is known to have survived a forty-six story fall, and there is even evidence of a cat that was deliberately thrown out of a Cessna aircraft at 800 feet that survived.