What is the biggest nuke on earth?

Answer 1

The "Tsar Bomba" was the biggest bomb ever detonated, at 50 megatons. The U.S has bombs ranging in yield of 0.01 Kilotons, to about 1 Megaton. The reason for 1 megaton, is that there is no real benefit of having them any bigger.

Answer 2

The largest nuclear weapon ever detonated was the Soviet Union's Tsar Bomba in 1961. It had an explosive yield of 50 megatons, making it the most powerful nuclear weapon ever tested.