There are 6 very strong acids: hydrochloric acid, sulphuric acid, nitric acid, hydrobromic acid, hydroiodic acid, perchloric acid and chloric acid.
The pH of the respective strong acids at 1mM is as follows:
HCl: 3.01
H₂SO₄: 2.75
HNO₃: 3.01
HBr: 3.01
HI: 3.01
pH = -log [H+], pH is the negative logarithm of the hydrogen ion concentration. The stronger the acid the lower is its pH.