While he definitely wasn't the first super-powered he-man to grace comic book stands (that honor goes to Doc Savage), Superman was unique because he was the first super-powered he-man to actively fight the injustices that plagued modern American society. He embodied the idea of Franklin Delano Roosevelt's New Deal. That it was the responsibility of the powerful to help those who needed it and punish those who deserved it. The fact that he did it in colorful tights and a bright showy cape gave him a sense of showmanship; like a wrestler or famous strong man at the circus or carnival. Every superhero after him echoes these sentiments. They're all altered variations of course, but Superman set the mold for this fantasy. The fantasy that under our mild-mannered exteriors we are unstoppable forces for good.
Who knew that a brash, opinionated, and often troublesome six-year old would be so enduring? What makes Calvin so memorable is that even though he was a child he was essentially an adult with adult opinions dealing with the limits imposed on him by modern society (often represented in the form of his parents, school, peers, and even babysitter). His release was his imagination (which only grew more impressive as the strip continued) and the interactions he had with his best friend Hobbes (who acted as a sort of representation of the values of the natural world). It wasn't that his parents, teachers, or peers were bad people (with the exception of school bully Moe) but they lived their lives by certain rules and expected him to follow them. He wouldn't of course, and no one could really give him a good reason for why he should. This often got him and Hobbes into trouble (not Hobbes so much because the world saw him as a stuffed tiger, but that wouldn't stop Calvin from trying to pin blame anyway). His struggles were always entertaining because they were struggles that readers, both child and adult could identify with.
With hits like Watchmen, Swamp Thing, V for Vendetta, and many others, writer Alan Moore has earned himself a nice niche in comic book history. Of all his creations though, Promethea stands out. She is essentially everything Wonder Woman could be but isn't due to DC management, and she represents the very message that can found at the core of Alan Moore's depiction of superheroes. She is a product of imagination (literally the character comes to life whenever her alter-ego begins writing about her, or drawing her- it depends on the host that possesses her power). It's a good representation of just where superheroes get their power from and how each creator brings something new to the character.
4. The Spirit
While superheroes represent our fantasy to save ourselves, sometimes people just need a good story with an appealing character to enjoy themselves with. Will Eisner's Spirit does that. A former detective the world thinks is dead is pretty much an open invitation for all sorts of shenanigans, especially if said detective continues fighting crime in a getup that's not as outlandish as the tights and cape that a superhero would wear but definitely stands out. With his faithful sidekick, Ebony White, the Spirit did indeed get into some memorable scraps and dealt with memorable people. It was all the fun of a superhero tale, but without the heavy restrictions that were often imposed on the characters with tights.
5. Usagi Yojimbo
Anthropomorphic animals are usually typecast as humorous characters, or at worst, an empty shell to begin a toy empire. Writer/artist Stan Sakai has proven time and again that this does not need to be the case with his ronin rabbit, Miyamoto Usagi or Usagi Yojimbo. Whether he's fighting bandits, corrupt warlords, or even the occasional demon, Usagi never disappoints. He embodies just what it means to be the perfect warrior. Which says a lot about a guy living in Shogun Japan, a time period that is often fantasized over like the European Middle Ages or the American Wild West. Usagi is a romanticized character who often finds himself in situations that real ronin probably found themselves in (though probably weren't always as skillful or fortunate in their outcomes). The fact that he's a rabbit only makes him stand out even more.