Of all the insane DC comic book villains, none are as famous or as dangerous as the Joker himself. After all, not everybody can become Batman's ultimate archnemesis, but the Joker has managed it.
What makes him so dangerous isn't his physical strength, but the fact that he's unpredictable, and his evil deeds have no clear reason or a pattern which makes it difficult even for the best detective alive to catch up with him. However, Marvel doesn't fall too far behind when it comes to crazy villains. Some of them are just trying to be as crazy as the Joker, while others are even more insane.
10 Trying To Be: Doctor Doom
Doctor Doom is the main enemy of the Fantastic Four and he has led the superfamily into trouble more than once. When he's not busy fighting the Fantastic Four, he spends most of his time ruling the country of Latveria, his domain which is one hundred percent controlled by Doctor Doom and his army of Doombots.
Doctor Doom is actually very intelligent, as he has demonstrated over and over again when he was able to defeat his enemies or lure them into traps. What makes him insane are his often over-the-top plans on how to control the rest of the world and defeat those who stand against him. Still, he's nowhere as unhinged as the Joker.
9 Crazier Than The Joker: Red Skull
The scary, but sometimes overlooked thing is you don't even have to turn to the comics to find a perfect example of horrifying insanity. The period of Nazism in Germany, the Second World War, and everything that happened during this period of time is one such example.
Marvel drew inspiration from this dark period of human history in order to creating one of its most iconic villains, the Red Skull, who became Captain America's main enemy. After all, who else should fight the ultimate Nazi than the symbol of freedom himself? As for the Red Skull, he's a brilliant strategist but his mind is so twisted that even Hitler thought the Red Skull was too unstable, and that's saying a lot.
8 Trying To Be: Venom
Venom definitely isn't a poster child for sanity, since his very existence of being connected to a human host leads to a dissociation of character and a perfect example of a split personality. But as far as alien symbiotes are concerned, Venom is still one of the saner ones, which may sound ironic, but it's true nonetheless.
His behavior greatly depends on his human host, and how stable or unstable the person is. Stronger personalities can control the symbiote to a greater degree, and they, therefore, remain saner, even though the symbiote would probably like for them to spiral into insanity.
7 Crazier Than The Joker: Madcap
This Marvel villain may not be as well-known as other villains on this list, but that doesn't make him any less crazy. After all, what else would you expect from someone whose nickname even contains the word "mad?" Madcap even has an origin story similar to one of the many alleged backstories of the Joker.
He was the victim of a chemical accident that changed him and also, unfortunately, killed his family which drove Madcap right into the arms of madness. He has the power of regeneration, which is bad enough, but he can also drive everyone else around him mad as well and he doesn't feel any remorse for what he's done to other people.
6 Trying To Be: Norman Osborn
Some might claim that Norman Osborn is actually crazier than the Joker, but that depends on the point of view, since Osborn, unlike the Joker, doesn't remain in his "insane setting" all day long. In fact, he can usually switch between both personas, sometimes so well, that he doesn't even remember what the other person was doing, or he doesn't know that he's actually the Green Goblin.
Talk about a split personality so great that even Venom would be green with envy. The Green Goblin has done a lot of bad stuff, like killing Peter's first love Gwen Stacy, and his ruthlessness and insanity make him a truly dangerous Spider-Man villain.
5 Crazier Than The Joker: Bullseye
There's no man more dangerous than the one who hurts and kills other people because he enjoys it and can't imagine ever stopping. That's also one of the main things that separate the superheroes from the villains in the comics: superheroes usually don't want to take human lives, and if they do it, they easily become antiheroes or even villains in the eyes of other heroes.
Bullseye is as dangerous as they come. He's an excellent shooter who could have done something else but he chose to become an assassin for hire because he likes killing others, and if that doesn't make him insane, nothing else does.
4 Trying To Be: Dark Phoenix
One of the saddest things about Jean Grey and her life is that Jean does not want to be a bad guy, unlike so many other villains on this list. In fact, she'd like to lead a normal life but her abilities that separate her even from most of the other mutants and her fellow X-Men don't really give her a chance to be the person she'd like to be.
And that's before Dark Phoenix comes along and destroys the last chance Jean had at a somewhat peaceful life. The Phoenix force is powerful enough to twist Jean and make her into something else, but at her core, Jean isn't as crazy as the Joker.
3 Crazier Than The Joker: Carnage
Just like Venom, Carnage deserves a spot on this list, since he's as insane as any human who is controlled by an alien symbiote could possibly be. But the thing is that Cletus Kasady was an awful human being even before he and the symbiote became one, and that's saying something.
Kasady was a serial killer who took serious pleasure in what he was doing, and when he became Carnage, he got an even better opportunity to wreak havoc and destroy lives, which is something that he enjoys above anything else. He's also more dangerous than the Joker because he is physically stronger and even the combined forces of Venom and Spider-Man were barely enough to stop him from killing them.
2 Trying To Be: Loki
Largely thanks to Tom Hiddleston's charismatic portrayal of Loki in the Marvel Cinematic Universe, many fans now do not consider Loki to be insane, let alone in any way similar to the Joker. But the two iconic villains actually share a few traits. Loki is called the God of Mischief, and the Joker, as fans well know, likes to play jokes and pranks on others, often with a deadly result.
Loki sometimes teams up with the superheroes, but most of the time, he does what he wants, and that usually includes making Thor's life much less pleasant. Then again, he isn't as murderous as the Joker, so at least he's got that going for him.
1 Crazier Than The Joker: Thanos
Thanos is one of the most popular and influential villains in the MCU, and in Marvel comics in general, and for a good reason. His plan to wipe out half of the life in the universe may seem like a well-thought and somehow logical, albeit twisted goal, but when you actually stop and consider it, it's simply horrifying and beyond insane, no matter how cleverly Thanos justifies it to himself.
And that's not the only insane thing that the Mad Titan has ever done. After all, no-one in their right mind would fall in love with Death herself, and Thanos, although he may not seem like it at first, is so much crazier than the Joker that if these two met, the Joker would probably hang up his business and retire.