I agree; I can understand why Superman is depicted as the most powerful, by the simple virtue of excelling very wide array of powers that makes him the most versatile of the bunch. He's extremely strong, he can fly, he's very fast, he's got ice breath, heat vision, x-ray vision, super perception, etc. He really is the best all-rounder.
Unfortunately, some of the more rabid fans want him to be a complete god that overwhelm everything physically just for the sake of it, which just reeks of fan wank. Those who justify that by saying that his enemies shouldn't even try to challenge him physically by finding alternate ways to challenge his character and morals conveniently forget that the very same applies to every other members of the League; furthermore, how does that justify why Superman needs to be overwhelmingly more powerful than everyone else? Making him overwhelmingly powerful compared to the rest of the League for the sake of it is frankly unjustifiable.
Diana should have comparable strength and superior combat training, but she lacks most of Superman's powers, i.e. flight, heat vision. Aquaman should also have comparable strength as well as control of the waters, but again he lacks most of Superman's powers. The Flash should be the fastest member of the League, bar none; if Superman can match his speed then it renders the Flash completely redundant. It's the Justice League, not Superman and his helpers.