Superman is no longer the most powerful superhero in existence, just the smartest of the strongest. He can be matched by the Shazam mortals, Captain Marvel and Black Adam, easily. Moreover, they can beat the shit out of him because Superman has a weakness to magic. Also, there is the murderous Superman-Prime who is stronger than Superman because he has no mental blocks to prevent him from tearing through 9-inch steel like paper.
Also, recently, there is an entire planet full of "Supermen" on New Krypton, none of them with mental blocks.
His weakness to magic also means that Wonder Woman's sword forged by Hephaestus can penetrate Superman's milimetre thin forcefield. Superman also has a weakness to high-tier psionics.
Give Superman a Green Lantern ring, and I agree, he would rule the bloody universe. One of the strongest wills in the universe and the ability to overcome great fear, would annihilate anyone. Also, since Superman's cognitive abilities are genius level and can think as fast as the speed of light, any Lantern would be hard pressed fighting him.
The only thing that makes Superman strong is his ingenuity, his ability to use his powers in new and interesting ways. He rarely powers up to maximum, as he refers to this state as "being in a world full of cardboard", unless he fights Darkseid or any of the New Gods.
/endspiel