Not more powerful than the Tsar Bomb: https://en.wikipedia.org/wiki/Tsar_Bomba
In principle, we can design weapons horrendous enough to crack this entire planet apart forever. Four billion years of life up in smoke (or out in debris, I should say).
The question of whether humans are the most intelligent life form on the planet is still under active consideration. :)
[1] The Pentagon's Brain by Annie Jacobsen
The largest confirmed explosion on earth was on the order of 1 teraton (1,000,000 megatons) caused by the Vredefort asteroid impact and that certainly left a crater but did not come close to cracking the planet forever. It left a crater 40km deep and 300km wide, which is absolutely enormous and mind boggling and if it happened today would certainly cause a mass extinction event, but the Earth would recover.
The point isn't the exact number, it's the fact that it's so reasonable to attain no matter the number.
The earth recovered its collision with the asteroid that eventually gave us the moon, yet the earth has actually been “taken apart forever” (since a significant fraction of its former mass now belongs to the moon).
That's a bit bigger than Chicxulub. Someone estimated the heat from the re-entry of debris at 20kw/m2 for 7+ minutes, not close up, but far away. A random study on firefighting I just skimmed said that heat flux will destroy clothing in about 20 seconds and produce 3rd degree burns in another 20 or so. So, uh...ten times 40 seconds over practically the whole world would probably not be good.
The collision that produced the Moon is speculated to have left the surface of the Earth approximately as bright as the Sun for a while, so it could be worse, but there's hardly any difference for the forms of life we can relate to.
Anerobic organisms deep in the crust would have an easier time of it.
He had a large book which contained the list of the numerous races throughout the galaxies that had developed intelligence, and the much smaller book that listed those races that had reached maturity and had qualified for the Galactic Federation.
In the first book, a number of those listed were crossed out; those that, for one reason or another, had failed. Misfortune, biochemical or biophysical shortcomings, social maladjustment took their toll. In the smaller book, however, no member listed had yet blanked out.
And now Naron, large and incredibly ancient, looked up as a messenger approached.
“Naron,” said the messenger. “Great One!”
“Well, well, what is it? Less ceremony.”
“Another group of organisms has attained maturity.”
“Excellent. Excellent. They are coming up quickly now. Scarcely a year passes without a new one. And who are these?”
The messenger gave the code number of the galaxy and the coordinates of the world within it.
“Ah, yes,” said Naron. “I know the world.” And in flowing script he noted it in the first book and transferred its name into the second, using, as was customary, the name by which the planet was known to the largest fraction of its populace. He wrote: Earth.
He said, “These new creatures have set a record. No other group has passed from intelligence to maturity so quickly. No mistake, I hope.”
“None, sir,” said the messenger.
“They have attained to thermonuclear power, have they?”
“Yes, sir.”
“Well, thats the criterion.” Naron chuckled. “And soon their ships will probe out and contact the Federation.”
“Actually, Great One,” said the messenger, reluctantly, “the Observers tell us they have not yet penetrated space.”
Naron was astonished. “Not at all? Not even a space station?”
“Not yet, sir.”
“But if they have thermonuclear power, where do they conduct the tests and detonations?”
“On their own planet, sir.”
Naron rose to his full twenty feet of height and thundered, “On their own planet?”
“Yes, sir.”
Slowly Naron drew out his stylus and passed a line through the latest addition in the small book.
It was an unprecedented act, but, then, Naron was very wise and could see the inevitable as well as anyone in the galaxy.
“Silly asses,” he muttered.
Isaac Asimov − Silly Asses (1958)
The ending (after the second war) referred to "half-stooping, naked things whose twisted heredity could have been traced to mankind." and said something about how, since they could be frightened, they were definitely not human any more.
It's what led to the development of the smart bomb - they figured out that it's a losing game to increase the power of a bomb - that cube root is painful. So instead they made small, but more accurate, bombs.
It's also why the nuclear bomb is not as useful as people think: It's strong, but a large number of accurate, small, smart bombs works better to destroy your target (as opposed to digging holes in the ground).
https://nuclearsecrecy.com/nukemap/
Pick a spot, the yield of the weapon, and it will draw radius circles for the fireball, ionizing radiation, 3 degrees of blast damage, and finally, the broad thermal radiation damage. I imagine there's others with things like fallout, etc.