The safest bet is I think hydroiodic acid (HI), with a pKa (from Wikipedia) of -9.3. The stronger acids are less well-known, the weaker common acids are less strong.
If the definition of "strong acid" is one which has a pKa higher than that of the hydroxonium ion (pKa=-1.74), chloric acid has inconsistent figures. Wikipedia lists -1 for the pKa of chloric acid, which would suggest it is not quite strong, and this may be why you lost the mark.