Which acid is stronger?

Well the magic seven are HCl, H2SO3,HNO3, HI, HBr, HClO3, and HCLO4; these seven are commonly known was the strong acids. you just have to memorize the "magic seven". However if you would have know the molarity you could have either taken the -log[H+] to get the pH. or just used the Kw=Ka times Kb formula, but for your purpose I think that you just needto know the magic seven.