Radon is pretty stable chemically, though. It does decay pretty badly, but as a noble gas, it's not too eager to form any sort of compounds. If it wasn't for radioactivity (which isn't on a record level, either, but sufficient that it isn't good for much), it'd be essentially a slightly heavier version of xenon.
Ozone, on the other hand, is worse than HTP, which is saying something. It just wants to revert to oxygen far too much, and will do so spontaneously, even as a liquid.
Depends on a mission. Uninsulated LH2 can be worse, though there are more ways of dealing with boiloff than with radioactive decay. It's a bad propellant, and not only because you lose half of it every 4 days, but pure ozone at pressures we're talking about is worse, because it explodes at random, and there's no way of solving that problem, either.
Radon could be potentially useful for high-thrust, unmanned, short-duration MPD missions, though it's a niche. Ozone can't be used because it'll explode even if all you do is purify it and let it sit in a pressurized tank for a while. Cosmic rays don't help there, either.