All credit goes to DeathstroketheHedgehog for these calcs. These are his, not mine.
Here is the command block setup used.
Creeper[]
Creeper Basalt Explosion Test
The creeper can destroy up to 99 cubic meters of basalt.
That's 99,000,000 cubic centimeters, and 2/3 of these blocks are pulverized.
66 million * 224 = 14,784,000,000 joules.
This is 3.533461 tons of tnt, or Large Building level
Charged Creeper[]
Charge Creeper Basalt Explosion Test
The Creeper, while charged, can destroy up to 478 cubic meters of basalt.
That's 478,000,000 cubic centimeters, and 5/6 of these blocks are pulverized.
398.33333... million * 224 = 89,226,666,667 joules
This is 21.325685 tons of tnt, or City Block level
Wither's Spawn Explosion[]
Wither Explosion Recalc (read description)
The wither's spawning process can destroy up to 839 cubic meters of basalt.
That's 839,000,000 cubic centimeters, and 6/7 of these blocks are pulverized.
(5034/7) million * 224 = 161,088,000,000 joules
This is 38.500956 tons of tnt, or City Block level
The Ender Dragon's Ramming[]
The Ender Dragon is capable of destroying up to 208 blocks of iron by ramming. This number is taken from the number of blocks the Ender Dragon can destroy using its whole body. 208 blocks of iron is 208,000,000 cubic centimeters of iron.
208 million * 60915.7 = 12,670,465,600,000 joules
This is 3.028314 kilotons of tnt, or Small Town level.