The Avengers most often face villains cribbed from one of their members' rogues galleries, such as Captain America's adversary Baron Zemo or Thor's evil brother Loki. However, there are two villains who exist primarily to challenge the Avengers as a whole: the genocidal android Ultron, and the time-traveling despot Kang The Conqueror.

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Both Ultron and Kang are formidable villains who have inflicted great pain upon the Avengers many times over, but which one reigns supreme as the true arch-enemy of Earth's Mightiest Heroes? Let's find out...

10 Ultron: Created By An Avenger

Ultron is the creation of founding Avenger Hank Pym, known by many identities over the years but most commonly as Ant-Man; Pym went as far to use his own personality as the basis for Ultron's. Pym's role in creating Ultron means that the Avengers share a degree of culpability for all of the android's atrocities, and makes their interest in stopping him all the more personal. The greatest Ultron stories likewise put his oedipal relationship with his "father" front-and-center, focusing on Pym's guilt for having given life to one of the Marvel Universe's most consistent scourges and delving into how Ultron is essentially Pym's inferiority complex made manifest.

9 Kang: Appeared First

Kang the Conquerer Cropped

Befitting the self-proclaimed "Master of Time," it's only fitting that Kang would be one of the first villains the Avengers faced. First appearing in The Avengers #8 and doing battle with the team's founding members, Kang quickly became one of the team's most recurring adversaries, returning for two multi-part storylines during the series' first 100 issues. No matter how the line-up of the Avengers changed, Kang was there to oppose. In contrast, Ultron debuted in The Avengers #54, by which point the team had already gone through several line-ups and battled dozens of villains.

8 Ultron: MCU Appearance

Ultron MCU

The Avengers have the Marvel Cinematic Universe to thank for their explosion in popularity among the general public; over the course of the 21st century, the team has gone from relatively obscure to the poster boys of the Marvel brand. Only one of their foremost enemies has transitioned with them to the big screen, however, in 2015's eponymous Avengers: Age Of Ultron.

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While the film changed some aspects of the character from the comics, Ultron was still the villain of a blockbuster film while Kang has yet to appear in any form within the MCU.

7 Kang: Contrasts The Avengers' Big Three

Villains should ideally reflect or embody an aspect of the hero, but inverted and corrupted. Kang does this beautifully to the three most iconic Avengers: Captain America, Iron Man, and Thor. Cap is a superhero Rip Van Winkle, a man out of time, whilst Kang is a man who can travel through time at will. Tony Stark is a futurist, a man who invents and uses his inventions in the hope they will bring a better tomorrow. Kang, on the other hand, is literally from the future. Then Thor, a living God who dwells among much-less advanced humans, while Kang is a from a point where humanity has advanced so far that he might as well be a God.

6 Ultron: Destroyed A Nation

Ultron with the Avengers lying in defeat behind him in Marvel Comics Ultron Unlimited

The Kurt Busiek-penned Ultron Unlimited, comprising The Avengers (volume 3) #19-22, is considered by fans as the greatest Ultron story ever told in comics. In the story, Ultron doesn't just cross the line of no return, he trail blazes right past it and burns the bridge along the way. The android slaughters the entire populace of Slorenia, intending to use the country as a staging ground for a more complete genocide, and to add insult to injury, he reanimates the corpses of his victims as cyborg-zombies, forcing the Avengers to destroy them in order to defeat him. While his ultimate plans were of course stopped, Ultron's destruction of Slorenia is one villainous act that never, and can't be, undone.

5 Kang: Successfully Conquered Earth

"Take over the world" is about as stock a villainous goal as can be, but despite the hundreds of villains, in comics or otherwise, who have attempted, the ratio of attempt to success is rather low. Kang thus towers above much of his villainous brethren, for in the 2001 Avengers storyline The Kang Dynasty, the master of time successfully conquered 21st century Earth.

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Finally conquering Earth in The Avengers (volume 3) #49, Kang became the first villain in the Marvel Universe to conquer present-day Earth 616, without the event being written off as an alternate timeline. While the victory was temporary, that he even temporarily conquered Earth is a testament to Kang's strength as a villain.

4 Ultron: Design

Let's be real here; if we're grading these two villains on pure aesthetics, than Ultron takes the cake easily. His body has a beauty-in-simplicity element to its design, a chrome silver-plated replica of a human body, but it's his head that makes the design work. Bearing a jack-o-lantern grin, with antennae along the side of his head alluding to his origins as Ant-Man's creation, and empty, menacing red eyes, Ultron's bears one of the most distinctive profiles among Marvel's villains. Kang, on the other hand, with his garish green-and-purple outfit, has always borne a distinctively  "comic-book" quality to his look, and not exactly in a good way.

3 Kang: Immortus

Kang the Conqueror with Vision and Human Torch

Kang the Conqueror's history is a long, tangled web, owing to his a ability to travel across time periods. As a result, many versions who have faced the Avengers have come from distinct timelines, to the point its a bit unclear which can be counted as the "true" Kang. One such alternate version even took up his own identity: Immortus, Lord of Limbo, a future version of Kang who changed his goal from preserving timelines rather than conquering them. Debuting in The Avengers #10 as a benefactor of the Masters of Evil, Immortus was revealed by writer Steve Englehart in Giant-Sized Avengers #3 to be an alternate Kang.

2 Ultron: Created An Avenger

Ultron's personal connection to the Avengers is twofold; he isn't just the creation of one of the Avengers, he created one of them, The Vision. Debuting shortly after his creator in The Avengers #57, The Vision was created by Ultron, using a brain scan from the then-deceased Simon Williams/Wonder Man, to lure the Avengers into their own destruction.

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Vision, of course, moved past his master's programming and sided with the Avengers, destroying Ultron's body (though his consciousness endured), becoming of the Avengers' most valued members, and fostering a relationship with fellow Avenger Wanda Maximoff/Scarlet Witch.

1 Kang: Iron Lad

Kang in his signature suit of armor.

Continuing the theme of Kang's tangled history, not all versions of the time-traveling warlord have been adversaries of the Avengers, for a younger version of Kang adopted an entirely different persona: Iron Lad. A founding member of the Young Avengers, Iron Lad is version of Nathaniel Richards (Kang's true name, and yes, he is a descendant of the Fantastic Four's Reed Richards) hailing from Earth-6311. Taken on a tour throughout history by his older self, the young Richards was horrified by Kang's atrocities and worked to avert them, thus becoming Iron Lad, whose first heroic act was to defeat Kang alongside his Young Avengers teammates.

NEXT: 5 Reasons The X-Men Have The Best Villains (& 5 Reasons The Avengers Do)