What is the strongest known acid?

Triflic acid is fairly dangerous, with ${\text{pK}}_{a} \approx - 14$. Anything with fluorides coordinated around it will increase a compounds acidity due to the dipole.
Fluoroantimonic acid is probably one of the (if not the) most acidic species, with ${\text{pK}}_{a} \approx - 31.3$.
Those hydrogens on that fluorine are extra nasty, they'll protonate even hydrocarbons I'd imagine. For comparison, sulfuric acid is a typical strong acid, with ${\text{pK}}_{a} \approx - 3$.