Remember in Aliens how the xenomorph's blood was acidic enough to burn through the ship? Yeah, well that's not even as bad as fluoroantimonic acid. If the aliens had fluoroantimonic acid in their blood -well, it would dissolve their whole bodies, but if it managed to stay in their bodies until they were shot, it would not only eat a whole through the thickest panels in the ship, but the potent vapors would also kill pretty much everyone on the ship.
While fluorantimonic acid is the most acidic substance on earth, it's only one of the ten crazy substances included in this TopTenz list of over-the-top insane substances. What's the most deadly poison? How about the most flammable substance? Or the most radioactive material? You'll have to find out at the link.