I was searching on reddit for powerful theoretical weapons, until I came across an interesting response:
"Confine a bunch of hydrogen ions in a very small space. Then release them. The force they experience due to electrostatic repulsion is mind-bogglingly big, so they're accelerated to very near the speed of light almost instantaneously.
So you have a sphere of plasma expanding outward near C. The rest of the picture basically paints itself. They exchange momentum with air molecules, the air molecules go fast and bang into other air molecules and pretty soon (like within microseconds) you've got a big expanding fireball. It would probably generate a mushroom cloud just like a nuke, too. The difference is that there wouldn't be fallout, although I haven't really crunched the numbers to figure out if fusion might take place, which would release some radiation. Like I said, the numbers you get from this are gargantuan, so it's very possible.
As far as pound-for-pound comparison goes, depending on how you approximate it, 10 grams of hydrogen confined in a 1cm sphere could yield anywhere from 100 kilotons to 1.5 megatons. The problem, I guess, would be figuring out how to ionise all that hydrogen before your apparatus blows itself apart, which incidentally, is also a problem with nuclear weapons. They only react a little bit of their fuel before they churn themselves into a puff of radioactive vapor.
This is all based on some calculations I did when I was bored a couple weeks ago. I didn't save any of the numbers and I can't use WolframAlpha right now and re-do it, but if you want to crunch them again yourself all you need is Coulomb's Law, F = ma, and the formula for kinetic energy at relativistic speeds, which I can't remember off the top of my head."
I wonder how all that hydrogen can be ionised, or how it can be contained, or if the concept even works at all.
RE: I personally think that to ionise the hydrogen, perhaps an electron capture can be used, maybe with an electron flash. And to store the ions, maybe an ultrastrong material, such as Alexander Bolonkin's Nuclear Matter can be used.