What atomic bomb did the most damage?

Answer 1

It was actually the lower yield MK-I Little boy bomb dropped on Hiroshima that did more damage.

The higher yield MK-III Fatman bomb was accidentally dropped much further from its target (almost 2 miles away) and the blast wave was contained by the surrounding hills.

Answer 2

It might be surprising, but the lower yield (roughly 13 kilotons) Little Boy Uranium gun bomb produced significantly more damage than the higher yield (roughly 22 kilotons) Fatman Plutonium implosion bomb.

This was due to the combination of how far the bombs missed their target by and the terrain around the target.

1. Little Boy was detonated only roughly 800 feet from its Aim Point and the terrain of Hiroshima is very flat, permitting the blastwave to travel unconstrained and producing maximum damage for the yield.
2. Fatman was detonated more than 1.5 miles from its Aim Point; in the Urakami Valley near Nagasaki instead of directly over Nagasaki and hills between the valley and Nagasaki contained much of the blastwave, preventing much of the possible damage the bomb could have done.

Answer 3

The MK-I bomb dropped on Hiroshima even though it was of lower yield did more damage to its target than the MK-III bomb dropped on Nagasaki. This was because Hiroshima is flatter than Nagasaki allowing the blast wave and thermal flash to spread further unobstructed and the bomb dropped on Nagasaki was mistakenly dropped 1.9 miles off target causing its damage to be largely contained inside the nearby Urakami valley. The total damage caused by the MK-III bomb may have been larger, but much of it was in the wrong place.