In "The Man Who Has Everything", wasn't Superman mad enough to kill Mongul? Mongul was still able to take what Superman was dishing out, and if not for Robin's interference, wasn't Mongul about to win their battle?
That's because back in the bronze age, DC gave writers their own creative freedom to portray supes as powerful (or as weak) as they like to be depending on the story..
Seriously take all of that silver age strength that juggles planets, and a sneeze that blows away solar systems..
And apply all those force in a single punch against Mongul?? no mongul would be dead.. no butchered
And no matter what speed mongul can compensate.. Supes didn't use a light speed onslaught..
Indicating that this alan moore written superman is written creatively free to be weaker.
Jim Shooter wrote a story in which "Solomon Grundy" oa all people KO'ed Supes... another proof of creative freedom..
But to see which is accurate, the strongest portrayal has to be the most valid bacause it showcases actual feats and their peak..
BTW Mongul can't fly....