The highest pKa value means the weakest acid. Why not look up the Ka or pKa values and see.
http://clas.sa.ucsb.edu/staff/Resource%20Folder/Chem109ABC/Acid,%20Base%20Strength/Table%20of%20Acids%20w%20Kas%20and%20pKas.pdf
Which of the following has the HIGHEST
pKa?
1. HClO3
2. HBrO
3. HClO
4. HIO
5. HClO4
2 answers
HClO3 and HClO4 are both strong acids, and therefore will not have the highest pKa. Between Cl, Br, and I the LEAST electronegative is I, meaning it wont pull the electrons towards itself as much as Cl or Br, so it wont weaken the H bond as much. HIO has the highest pKa.