What is the largest nuclear bomb?

Answer 1

The Tsar Bomba. It was tested in 1961 by the USSR on the island of Novaya Zemlya within the Arctic Circle. Its design yield was 100 megatons, but the yield was reduced to 57 megatons for the test by replacing the uranium-238 third stage tamper with a different metal (e.g. lead, tungsten) that would not fission when hit by the high energy fusion neutrons (and thus would not contribute to the yield or the fallout).

The Tsar Bomba was not only the highest yield nuclear bomb ever tested, it was the highest yield nuclear bomb actually dropped from a bomber airplane. The Tsar Bomba also turned out to be the cleanest nuclear explosion with only about 3% fission yield and a correspondingly small amount of fallout relative to its total yield. Nuclear explosions having 5% or less fission yield are usually called "clean".