The Hulk is one of Marvel's most popular heroes, with Mark Ruffalo's portrayal of the conflicted monster making Bruce Banner an enduring part of the MCU. Many of Hulk's most iconic villains have made the journey to the Marvel Cinematic Universe, including his chief nemeses the Abomination and the Leader. However, the movies have yet to touch some of Hulk's strongest opponents.

Here are Hulk's 12 strongest villains who have yet to appear in the MCU. We're being strict with this list and trying to include even questionably-canon sources, which means that Absorbing Man's role in Agents of S.H.I.E.L.D. disqualifies him from consideration, whether or not Phil Coulson's team are technically considered modern MCU canon.

12 The U-Foes

Debuted in Incredible Hulk #254 by Bill Mantlo and Sal Buscema

An evil version of the Fantastic Four, the U-Foes have a grudge against Hulk for messing with the experiment that was meant to give them the same powers as the iconic heroes. Vapor, Vector and Ironclad boast a range of powers capable of giving the Hulk trouble, from Ironclad's strength to Vapor's ability to suffocate Bruce Banner in her gaseous form. However, the biggest threat is X-Ray, who is able to bombard Hulk with cosmic rays - the opposite of gamma radiation and one of the few forces that can kill him.

11 The Red King, aka Angmo-Asan II

First Seen in Incredible Hulk #92 by Greg Pak and Carlo Pagulayan

By rights, the Red King should have appeared in Thor: Ragnarok, given that in the comics he's the tyrannical ruler of the planet Sakaar. Sadly, his place was taken by Jeff Goldblum's Grandmaster in the semi-adaptation, meaning fans didn't get to meet the egotistical villain. The imperial ruler of Sakaar, the Red King was a solipsistic ruler whose father never wanted him to take the throne. Sadly, the Red King was able to take power, persecuting the various species of the planet and setting up vicious gladiatorial contests for his entertainment.

Thankfully, when the Hulk came to Sakaar, he deposed and defeated the Red King, though the villain unexpectedly survived, converted into a cyborg by the planet's rogue AI species the Wildebots. Prior to this, the Red King fought using a mech suit that enhanced his strength and durability, as well as containing weaponry including a flamethrower and energy shields. The Red King was able to hold his own in direct combat against the Hulk using the suit, though this was while he was able to incapacitate the Green Goliath using a surgically implanted obedience disc.

10 The Rhino, aka Aleksei Sytsevich

Debuted in Amazing Spider-Man #41 by Stan Lee and John Romita Sr.

While the Rhino is a classic Spider-Man villain, he's also a recurring antagonist of the Hulk, and the two even share origins related to gamma radiation. While the Hulk's DNA was bathed in the quasi-mystical energy, Rhino uses it to treat his super-durable suit, enhancing his strength without making him a true 'Hulk.' Rhino possesses incredible strength which is enhanced by his surprising speed, often targeting the Hulk to prove his might.

While Rhino hasn't appeared in the MCU, he was played by Paul Giamatti in 2014's The Amazing Spider-Man 2 and by Alessandro Nivola in the recent Kraven the Hunter.

9 Wendigo

Debuted in Incredible Hulk #162 by Steve Englehart and Herb Trimpe

The flesh-eating monster known as the Wendigo is more of a curse than a single foe, with anyone who consumes human flesh within Canada's borders becoming a gigantic beast. Hulk has fought many different versions of the Wendigo, and X-Men's Wolverine was even introduced during a massive Hulk vs Wendigo brawl. Wendigo is strong enough to hurt Hulk, even if they're not quite in the same weight class, but the monster's speed and claws make it a threat Bruce Banner never takes lightly.

8 Mercy, aka Abigail Wright

Introduced in Incredible Hulk #338 by Peter David and Todd McFarlane

An alien who doesn't fully understand human emotion, Mercy is one of Hulk's most enigmatic and terrifying villains. Mercy considers herself a mercy killer, slaughtering anyone who is struggling with negative emotions. She has enhanced strength and durability as well as the powers of flight, pyrokinesis, telepathy and intangibility. Whenever Mercy senses strong negative emotions, she will do her best to incinerate the person feeling them, believing she is freeing them from suffering.

Because of Hulk's rage, Mercy has targeted him again and again, trying to 'free' him from anger and sadness. However, thanks to his durability and healing factor, Hulk has always been able to survive Mercy's attacks, turning him into a figure of obsession for the dangerous superhuman.

7 Xemnu

First Seen in Journey Into Mystery #62 by Stan Lee and Jack Kirby

Xemnu is an even older character than the Hulk, debuting two years earlier in 1960. An evil alien who has enhanced his body with cyborg upgrades, Xemnu is one of Marvel's most powerful telepaths. With immense physical strength and telekinetic powers, Xemnu feeds on the minds of his victims and can turn them into zombie-like thralls. Hulk and Xemnu have clashed several times, but their most noteworthy fight was in Al Ewing and Joe Bennett's Immortal Hulk, where Xemnu attempted to use its hypnotic powers to edit the Hulk out of human history and replace him.

Xemnu attempting to replace Hulk was a reference to the fact that the character was originally named 'Xemnu the Hulk,' before the title became more famous as Bruce Banner's codename.

Xemnu has a particular grudge against the Hulk after several clashes with the Defenders, but also because Hulk's unique memory means that he can't be brainwashed as easily as regular humans, making him a constant problem for Xemnu's invasion attempts.

6 Juggernaut, aka Cain Marko

Debuted in X-Men #12 by Stan Lee, Jack Kirby and Alex Toth

With the X-Men poised to enter the MCU, it may not be long until their old enemy the Juggernaut appears in full MCU canon. Professor Xavier's bullying step-brother, Cain Marko gained powers after pillaging an ancient gem during the Korean War. Touching the gem connected Juggernaut to an eldritch being known as Cyttorak, who rewards him for the destruction he wreaks in the demon god's name.

Juggernaut possesses superstrength slightly less powerful than the Hulk, but makes up for the deficit with his 'magic momentum,' which makes it impossible to stop his forward momentum once he reaches a certain speed. In recent years, Juggernaut has turned hero, working with the X-Men, however the Hulk will always consider him a rival thanks to their history.

Juggernaut has appeared in multiple movies, including X-Men: The Last Stand (played by Vinnie Jones) and Deadpool 2 (voiced by Ryan Reynolds.)

5 Man, aka Philip Sterns

Debuted in Incredible Hulk #362 by Peter David and Jeff Purves

While Samuel Stern's Leader appeared in both The Incredible Hulk and Captain America: Brave New World, his brother has yet to enter MCU continuity. Philip Sterns is a rival of both his brother and the Hulk, with a backstory as a significantly less successful scientist who envied Bruce Banner's accomplishments and career. Hoping to equal Bruce, Philip exposed himself to gamma radiation, becoming a towering red powerhouse known as Man.

The gamma exposure affected Philip's mind, resulting in constant paranoia and a desire to control the Hulk, whose strength he's capable of equaling and even suring, albeit in short bursts of power.

4 Zzzax

Debuted in Incredible Hulk #166 by Steve Englehart and Herb Trimpe

Zzzax is an energy being who feeds on electricity, often attacking humanity in an attempt to prolong its life. Near-mindless, Zzzax is capable of solidifying its form, wielding incredible levels of strength. However, because it doesn't have any true 'body,' Zzzax is impossible to injure or kill, leading the monster to return again and again over the years.

3 The Guilt Hulk, aka Brian Banner

First Seen in Incredible Hulk #267 by Bill Mantlo and Sal Buscema

Easily Hulk's most underrated villain, Brian Banner is Bruce's father. In life, Brian Banner was a horrendously abusive father and ultimately murdered Bruce's mother, Rebecca. Bruce later killed his father in a supposed accident, allowing Brian access to eldritch forces that made him an even bigger threat. Brian has taken multiple forms since, often tapping into gamma radiation. During the Chaos War event, he manifested as a gigantic Hulk-like monster, while in Immortal Hulk, he possessed multiple gamma beings including the Hulk and Sasquatch.

Hulk also has an alternate persona known as the Guilt Hulk which is seemingly Bruce's traumatic perception of Brian - though given Brian's later body-stealing abilities, there's reason to believe Bruce's father may be able to influence the Guilt Hulk in ways fans didn't suspect until recently. Brian later melded with Marvel's ultimate evil, the One-Below-All, allowing this terrifying force of evil to express itself using his form and memories.

Brian Banner nearly made it into the MCU thanks to being mentioned in a cut scene in Thor: Ragnarok. He was more successful in Hulk's pre-MCU Marvel movie, with a version of Brian named 'David Banner' acting as the main villain in 2003's Hulk, played by Nick Nolte.