I'm going to go in a slightly different direction here and name one of my favorite stories instead of one that would objectively be one of the "best". It's a kind of goofy imaginary story from 1963, that to me exemplifies the silly sort of charm the Silver Age Superman had. It's the story of Superman Red/Superman Blue from Superman #162 - note this is NOT the story of the same age from the '80s.
Basically, Superman invents a machine to make him smarter so he can solve all the problems of the world and his life. It works but it has a side effect of splitting him into two, one wearing a red costume and the other wearing a blue (figure out the science of that!). Between the two of them, they set out to solve all the problems. It's light entertainment, but it's always stuck in my head.