Stronger Acid
It is perchloric acid, HClO4.
Perchloric acid is generally regarded as the strongest of all known simple acids.
It is stronger than any of the common hydracids and oxoacids such as HF, HCl, HBr, HI, HNO3, H2SO4 and H3PO4.
Hydrochloric acid (HCl) is even weaker than hydrobromic and hydroiodic acids (HBr and HI ).
To determine the acidic strength of any acid we first determine the the stability of its conjugate base.
Refer the picture of a model of CLO4 in Google images, as per VSPER theory. You would find 3 doubly bonded oxygen and one single bonded oxygen carrying a negative charge, now we have to prove that it is a strong acid, ofcourse I won't take away all your fun,
First find the no. of resonance structure s possible for it and the no. of bonds it has, find the bond order. Bond order is a direct indication of stability.
The answer you should get is 7/4, (7 bonds/ 4 resonating structure s). This explains its high stability, not only that since it is a non metallic acid it would be even stronger acid as it has high polarising power owing to its high non metallic character, and also the +7 O.N which it has further boosts it to Tremendous amount.
To explain you this, you can take oxides of Mn which is Mno, MnO2 , Mn2O7. Now, you know that manganese is supposed to be a metal and should form basic oxide as the convention goes yet Mno is basic, MnO2 is amphoteric(if my memory serves me right) and Mn2O7 IS ACIDIC, why? Because of the +7 O.N. it's polarising power made it so.
Now, do you really wanna compare HCl with this?, I mean sure Cl^- is stable, but HClO4 is on a whole different level.