While there have been a number of very large blasts used in mining, and accidents in making and transporting explosives, the largest non-nuclear explosion (not counting volcanoes) Was probably the spectacular accidental explosion of the N-1 rocket in the former Soviet Union in July 1969. The explosion was the equivalent of between 6-7 thousand tons of TNT.
The largest nuclear weapon ever tested, the Tsar Bomba by the Soviet Union in 1961, had a blast radius of over 7 miles. The fireball created by the explosion measured around 2 miles in diameter and the shockwave was felt over 400 miles away.
There is no one best gun, but the one with the largest explosion is the RPG if you are looking for something like that
A nuclear explosion can be millions of times more powerful than the largest conventional weapon. For instance, the largest conventional bomb, the GBU-43/B MOAB, has a yield of about 11 tons of TNT, while the most powerful nuclear bomb ever tested, the Tsar Bomba, had a yield of approximately 50 megatons of TNT. This means a nuclear explosion can surpass conventional weapons in destructive capability by a factor of thousands to millions, depending on the specific devices being compared.
No, a nuclear explosion on a nuclear power plant would not cause the explosion radius to increase. The explosion radius would be determined by the yield of the nuclear weapon itself, not by the presence of the power plant.
You could kindly refer to the links below for the images of nuclear weapon designs and their explosion consequences.
The explosion of an atomic weapon can cause an EMP.
The Soft Weapon was created in 1967.
Magic Weapon was created in 2005.
Raise Your Weapon was created in 2010.
The Weapon Makers was created in 1952.
The Weapon Shop was created in 1942.
Holy Weapon was created in 1993.