Flash might have a faster punch, but Supes has immense super strength along with the speed. Flash might send you flying with a headache, but Supes will send you flying HEADLESS. That's just my two cents
Come back after you read this.
However, I've watched documentary where they tested the punching strength of the strongmen (they're not martial artists) and when the force gauge reading come out, the documentary crews stated that they "punched harder than most heavy weight boxer." In human's experience, human can't build up as much punching speed to make significant difference compared to the mass of the fist.
7.63 x 39 mm Soviet is about twice as heavy as 5.56 x 45 mm NATO, it is 30% slower but can delivers more kinetic energy against target even with noticeable speed difference. The difference between humans are smaller than that.
Anyway, in comic, feats > actual facts, Superman throughout his career have shown that he can punch roughly the same magnitude as Flash and sometimes even harder. Not to mention Flash's IMP is Flash's maximum force, while many of the similar achievement have been done by Superman while he's holding back. Mr. Majestic have clearly and consistently shown to be faster than Superman in both combat and travel speed, and he does not hold back his fists like Superman does, but Captain Atom commented that Mr. Majestic's punch is not "Superman-strong."