The DC Universe-First of all, aliens are real. Very real. One of them embodies the best of humanity (which is ironic, because he's non-human). Some are good aliens (like Starfire of Tameran) and others are hostile alien overlords who want to conquer our planet and make it part of their empires (Mongul, Trigon the Terrible, and the Daxamites (briefly). Then there's actual supernatural figures as well...the Greco-Roman gods, the Hindu gods, and even to some degree some of the Supers, such as Superman. Then there's the Anti-Christ who decided not to be the Anti-Christ and is affiliated with a superhero team that is an Evangelion-style dysfunction junction made real. And there are also normal superheroes like Nightwing, Green Arrow, and Batman. And now we come to the forces of evil as humans define it. There's Doomsday, a psychotic Hulk-without-a-Banner, there's the Joker, who never gets convicted for what would in a real-world situation get him shot, and possibly Batman, too, there's Doctor Light who's just asking to get plugged.....and the real kicker is that the existence of other worlds is also real and that these worlds are linked by cosmic horror eldritch abominations, and you're almost guaranteed thanks to magic Comic Book Time to live through a period when everything you know is destroyed by hostile forces even the heroes of your planet cannot stop, which would mean psychotherapy would be immensely popular (and religion would rapidly resurge, possibly in a much-uglier form).
The Marvel Universe-Again, aliens perfectly real. There are powerful aliens that go into physical god territory like Galactus that eat planets with living beings on it, whom no hero can actually stop. Then there's people like Victor von Doom and briefly Magneto, who are rulers of a state who are both hostile to others and superpowered. Then you've Designated Heroes like Tony Stark, who are often difficult to distinguish from villains. The delightful side? Superman's just a really powerful alien. On this planet, pagan deities walk among us and engage in costumed vigilantism. Not to mention that a scientist who can do anything he wants is incapable of solving issues like world hunger and poverty. For bonus shits and giggles, the new version of the human race is evolving, one with superpowers, and humans are in the position of the Neanderthals 40,000 years ago: still present and strong, but we're already doomed. One of these mutants could warp reality at any time. Yippee-ki-yay.
Marvel supervillains tend to run the gauntlet of megalomaniacal (like Doom) to cosmic horrors that cannot be stopped, only delayed, like Galactus. Oh, and thanks to comic book time, every so often, the regular order of things is suspended by hyperpowerful entities that threaten life, the Universe, and everything.
The Wildstorm Universe-The Authority. Nuff said.
Heroes: Fettered monsters.
Villains: Complete monsters.
Which would you choose to live in?