When it comes to the most "anti" of all antiheroes, not many can come close to Spawn. Todd MacFarlane's brainchild and Image comics character dabbles in hellscape, war, social issues, and other dark themes. As such, its central character, Spawn (Al Simmons) is a disfigured undead or demon who's pretty much unmatched when it comes to comic book edge.
Okay, scratch that last part; there are beings who can match Spawn when it comes to advanced edge, mainly his rogues' gallery. These guys are hardcore and consist of some of the most soulless criminals and disgusting demons in comic books ever. Together with Spawn, they offered a nice change of scenery from the camp of the 1970s and 1980s. Of course, they are dangerous as heck and would make some of the more popular superheroes faint. For Spawn, they're just his regular weekdays despite the dangers they pose. Anyway, here are the most dangerous of them all, one for each of Spawn's weekdays and them some more.
10 CHAPEL
Before Al Simmons became Spawn, he was a dog of war-- a mercenary with an irredeemable soul and a high kill count. He was dead set for hell anyway but got the worst punishment when he was betrayed by his comrade, Bruce Stinson. Stinson helped murder Simmons and pretty much catalyzed his becoming Spawn.
Stinson, upon finding out he's no match for his now resurrected colleague, Spawn, committed suicide and was brought back as Lord Chapel. This time around, he worked for the Devil and was willing to betray and annihilate more of his former mercenary teammates. One thing's for sure, you don't want this guy on your team.
9 BILLY KINCAID
Billy Kincaid is no hellspawn or demon-- he's just a regular-looking human being which makes him a lot worse. His crimes are some of the most deplorable in Spawn's universe: he's actually a serial child killer. Kincaid has killed so many children, that Spawn was hired to undo him and his spree.
Spawn failed, though, and the police got to Kincaid first. Because of his mental condition, however, he got off easily and was only placed in an institution. After serving time there, Kincaid immediately went back to old habits and went on another child killing spree. He even used deceptive methods such as posing as an ice cream man to lure hapless children. Even as Spawn killed him (twice, one on Earth and the other in hell), Kincaid's ghost came back to terrorize humans.
8 OVERTKILL
Like most of Spawn's enemies, Overtkill was once a human and also had a dark past. He was similar to Al Simmons and his original name was Nicholas Rocca. Like Simmons, Rocca is a trained and seasoned killer, an assassin, which isn't much different than a mercenary. Rocca also died but returned as a cyborg rebuilt and revived with the murderous capability of whole armies.
He then renamed himself Overtkill and went on a rampage across the United States, killing everyone in his path. Once he knew of Spawn's existence, he then killed some more in order to lure him out of hiding just to have a chance to fight. Thankfully, Spawn ended him once and for all before he could add more bodies to his kill count.
7 MARGARET LOVE
Margaret Love is one of Spawn's few female enemies who have proven to be more dangerous than the likes of Billy Kincaid. She goes under the guise of a philanthropist who helps the poor and the homeless in cities but is actually a Soviet agent named Nadia Vladova. Turns out, Nadia was using her program for the homeless to lure them into being a part of her gruesome cyborg experiment.
Nadia apparently has a mad scientist fantasy of creating a cyborg weapon using the severed human heads of all the homeless people she tricked. Oh, and her solution for human greed is to start a nuclear war; because that's going to fix everything according to her. Thankfully, she only appears as a one-shot villain for the Spawn-Batman crossover comic book.
6 JASON WYNN
Like Kincaid, Jason Wynn is no supernatural being and is just a regular human being. However, that doesn't mean he isn't capable of wanton evil; Wynn serves as the director of the United States Security group, an umbrella department for the Central Intelligence Agency (CIA). Basically, he pulls the CIA's strings and is considered as the most powerful man in the country or even the world.
As you can imagine, Wynn's a master manipulator and keeps tabs on all things. He's the primary person responsible for Al Simmons' death since Simmons started asking questions about his dirty mercenary work. Without a doubt, Wynn is responsible for countless atrocities involving paramilitary factions. If that's not evil enough for you, Wynn also made a literal deal with the devil; he exchanged the soul of his best soldier (Simmons) for a mere token from hell itself.
5 THE VIOLATOR
The Violator is perhaps, one of the most common enemies of Spawn; he has no shortage of appearances in the comic books. The Violator is a direct agent of Hell and is best described as a lesser demon compared to the bigger evils that inhabit the place. He often goes to Earth to conduct the will of his masters but usually gets sent back to the deepest circles of Hell for failing.
That's because Spawn keeps meddling in his affairs, meaning the two encounter and fight each other frequently. When not in his true demon form, The Violator can be seen disguised as a fat middle-aged man with a creepy clown makeup. Funny enough, the Violator can't kill Spawn or Al Simmons without a direct order from his higher-ups. After all, his sole purpose was to groom Hellspawn for their roles in hell. Still, that didn't prevent the Violator from going out of line, going on a killing spree, and attacking Simmons.
4 THE REDEEMER
Since Spawn is technically an agent of hell, heaven isn't exactly warm to him. In fact, the forces of heaven have even sent plenty of angels to deal with Hellspawn, and Spawn happens to be on the Angels' hitlist. When Spawn became too much for their Angels, however, heaven created and sent a deadlier foe to dispatch the hellspawn: the Anti-Spawn.
Essentially, the Anti-Spawn is heaven's very own version of the hellspawn and the most notable among them is The Redeemer. Only the purest on Earth can become the Redeemer's host. Ironically, their methods are not exactly kind to mortals. Since their sole purpose is battling the Hellspawn, they usually go through extreme lengths to do it. At one time, the Redeemer even attempted to kidnap one of Spawn's friends just to lure him out; afterward, he nearly defeated Simmons but stopped only because the hellspawn also took an Angel as a hostage.
3 URIZEN
Urizen was one of the oldest demons in Spawn's universe; it was even described to be older than time, probably even older than Satan. For some reason, both the forces of heaven and hell imprisoned him in the void. It's not often a creature like that poses enough threat to convince both heaven and hell to cooperate, meaning Urizen's power could be vast. It is even surmised that it cannot be killed, only contained.
Oddly, enough, it doesn't take much to free and summon Urizen. A small sacrifice of 13 of his fanatics under a full moon was enough. When he was freed, Urizen caused lethal chaos on any town he passes, making the residents kill one another under his influence. Spawn tried to challenge and stop him but was defeated and killed easily. Lucky for him, he got resurrected with the power of Greenworld which allowed him to imprison Urizen once again.
2 MALEBOLGIA
Let's not forget Spawn's arch-nemesis and one of his original enemies: the greater demon Malebolgia. It's the demon who tricked Simmons into becoming a Hellspawn just so he could see his wife again on Earth. Malebolgia was Spawn's primary target and the one who caused so much pain and suffering for mortals involving Hellspawns. He was the "devil" whom Jason Wynn dealt with.
As for how much dominion it has over hell, Malebolgia actually ruled hell in Satan's stead and acted as its commander-in-chief until Satan returned. As such, Malebolgia is a respected ruler and authority in hell, even if there are other demons with the same power or capability. Moreover, Malebolgia is the Violator's overlord and almost everything that lesser demon does is through Malebolgia's behest.
1 SATAN
Despite being the acting ruler of hell, Malebolgia was only playing second fiddle to a greater evil; that is hell's progenitor itself, Satan. It was Satan who orchestrated the plans of his greater demons and other hell minions. All of that is in preparation for his war with God (in Spawn's universe) for control of Earth.
No other creature in Hell can match Satan in power, apparently, save for the Omega Spawn who many don't consider as canon since his storyline was never finished. Regardless, the only being in Spawn's universe that can match Satan's power is its counterpart in heaven or their creator, Mother of Existence.