The Squadron Supreme was created by Roy Thomas and John Buscema all the way back in 1971 as a foil to the Avengers. Though, something about all of its members seemed familiar. Hyperion had super strength, could fly and shot heat from his eyes; Power Princess was a strongwoman of mystical origin; Whizzer (yes, there's a hero called "Whizzer") ran with super-speed. You don't need any more examples to figure out that these were all analogues of DC's Justice League.
The Squadron had its share of strong stories in the main Marvel universe, including classic Avengers tale "The Serpent Crown." But it was in the mid-80s that they had their crowning moment. Set in their own universe, Mark Gruenwald spins a yarn about the group getting fed up with fighting off world threats and attempting to set the Earth straight for its own good. Very quickly, the concept of sacrificing freedom for protection enters the picture, driving a few members away from the team including Nighthawk (The Batman of the SS, only this "Bruce Wayne" stopped being a rich vigilante and decided to put his money towards helping an entire country instead of just a street corner.) What happens next, though, puts this story over the top.
The diminutive-in-size Tom Thumb (kinda like The Atom, but not really) attempts to cure the world of crime by creating a machine that eliminates illegal thoughts like murder, rape and theft as an alternative to the prison threat. While the ramifications of that alone are sizable, things get more up close as Golden Archer (Green Arrow, and yes... there's a hero called "Golden Archer,") shocked by a refused engagement uses it on Lady Lark (Black Canary) to make her fall back in love with him. Faced with being found out, Arrow... er... ARCHER leaves the team in disgrace. The Squadron replaces the void in their ranks by altering the minds of their own arch-villains and having them join.
Meanwhile, Nighthawk forms a team of villains and the two sides come to a head. The result is something you didn't see really at all in Superhero comics at the time: a bodycount. A lot of characters die in the climactic battle, let alone those who pass before it in the story's various subplots and the follow-up graphic novel "Squadron Supreme: Death of a Universe" which is also included in this omnibus.
Now, I wouldn't dare to say "Squadron Supreme" is better or even as good as "Watchmen" or "Kingdom Come." However, because it concluded only a month before "Watchmen" started its run that changed comics forever, it almost feels like it's been swept under the rug. I know many a fan who have read those two DC classics. I am confident they would appreciate "Squadron Supreme" a lot, especially considering that it came first.
Gruenwald certainly thought a lot of it; the ink from the first trade paperback contained his ashes.