The Avengers and the X-Men each represent two of the most powerful gatherings of heroes in the Marvel Universe, which often brings them up against each other in epic events like Avengers vs. X-Men that thankfully led to friendlier relations between the two teams.
However, it isn't just war between each other that brings the Avengers and the X-Men together, as they have been forced to team-up a number of times over the years in order to deal with immensely powerful threats that threaten their friends, their home, and their universe.
Updated December 14th, 2020: The current state of the Marvel Universe has the Avengers and the X-Men operating very independently from each other, as the X-Men have declared their sovereignty on Krakoa while the Avengers have set up in a new secluded headquarters at Avengers Mountain. However, there continue to be a number of powerful threats that still manage to bring the two teams together. From cosmic reality-threatening events to hostile national takeovers, the contentious allies are able to put any differences aside when it comes to protecting Earth from some of Marvel's worst supervillains— and even sometimes, closest allies.
15 Soviet Super-Soldiers - The X-Men Vs. The Avengers
After Magneto's former hideout, Asteroid M, crashed to Earth in the form of fiery debris, the Avengers attempted to draw out the redeemed villain in order to take him in for his past crimes in 1987's The X-Men vs. The Avengers.
The X-Men arrived to aid the Avengers, though the powerful Soviet Super-Soldiers (who would later become the Winter Guard) also arrived to execute Magneto. The Avengers teamed up with the X-Men to take on the Soviet Super-Soldiers, leading to Magneto voluntarily cooperating with the Avengers for his trial with the World Court.
14 Hydra Supreme - Secret Empire
Secret Empire featured a Hydra takeover of America that was led by none other than Steve Rogers/Captain America, who had been cosmically rewritten by the sentient Cosmic Cube known as Kobik to become Hydra Supreme.
While the X-Men didn't play the largest role in the Avengers-focused event, a number of them worked alongside Steve Rogers to protect the new mutant territory called New Tian, while others actively worked with other Avengers heroes in the resistance. New Tian, the remaining X-Men and the Avengers all formed together to help stop Hydra Supreme and his army.
13 Kang - Planet X
The time-traveling conqueror known as Kang was a frequent opponent of the Avengers over the years, though when they teamed up with members of the X-Men to form the Unity Squad, Kang set his sights on new targets.
He first kidnapped the recently born Apocalypse Twins (Uriel and Eimin) and took them to the future to raise them while also manipulating events in the past. However, the Apocalypse Twins turned on Kang and he was forced to briefly ally with the Unity Squad before attempting to turn on them to gain cosmic power, though his plan was stopped.
12 Beyonder - Secret Wars
1984's Secret Wars saw the incredibly powerful being known as the Beyonder transport a number of heroes and villains across the galaxy to his created planet of Battleworld in order to hold the ultimate contest of good and evil.
There were a number of separate conflicts in the event that included a battle between the Avengers and the X-Men with Wasp nearly taking out the entire team. The X-Men kept to themselves for most of the event, though they allied with Captain America's forces against the Beyonder, who was still manipulating events even after Doom stole his abilities.
11 Red Skull - AXIS
Following the death of Professor Xavier, the Red Skull stole his body and grafted Xavier's mutant brain onto his own, granting him access to vast telepathic abilities that the Avengers and X-Men's Unity Squad assembled to defeat.
Red Skull used his newfound abilities to incite the population into hateful and riotous acts, and he eventually evolved into the powerful Red Onslaught form. The Avengers and the X-Men had to fight inverted members of their own squad while also dealing with dangerous Stark Sentinels during AXIS, but they were able to work together and stop Red Skull.
10 Magneto: Dark Seduction
While Magneto is currently one of the leaders of the X-Men and has been with the team for years, in his earliest days he was the X-Men's greatest enemy whose goal of mutant superiority brought him into conflict with both teams many times.
Their earliest battles often featured a clash between the X-Men and the Avengers before they teamed up to take on Magneto, who was once able to take down an entire team of Avengers on his own in Magneto: Dark Seduction.
9 Exodus - "Bloodties"
The X-Men had been dealing with the civil rights issues of the mutant island of Genosha for years before the Avengers got involved after the Acolyte Fabian Cortez kidnapped Quicksilver's daughter Luna in the "Bloodties" storyline.
He attempted to use her as a bargaining tool in order to protect himself from Magneto, however, it was the mutant Exodus who proved to be the real threat, resulting in the Avengers and the X-Men joining forces to stop Exodus and save young Luna, though they really only managed to send Exodus back to Avalon for a time.
8 Cable - X-Sanction
While the militaristic mutant known as Cable can't really be classified as a villain, he also wasn't acting much like a hero when he attempted to take out the Avengers in the X-Sanction prequel to Avengers vs. X-Men.
Cable had come back from the future hoping to stop the Avengers from harming his adoptive daughter Hope Summers, and he took out the team one-by-one until Cyclops and Hope Summers of the X-Men arrived, freed the Avengers, and stopped Cable.
7 Scarlet Witch - House Of M
Scarlet Witch was the original bridge between the two teams as both a mutant (at one point) and a longtime member of the Avengers, so when her reality-altering powers threatened reality, the New Avengers and the X-Men came together to deal with her.
Unfortunately, thanks to her brother Quicksilver's manipulations, Wanda created the House of M reality and forced the assembled heroes to abandon their best life in order to rewrite reality, though it resulted in Wanda decimating the mutant population with one last spell— "No More Mutants."
6 Doctor Doom - Avengers: The Children's Crusade
While Doctor Doom was introduced as a Fantastic Four villain, he quickly became a threat to the rest of Marvel's heroes, with the Avengers and the X-Men dealing with him separately quite a few times over the years.
During Avengers: The Children's Crusade, Doom revealed to a grouping of Avengers, Young Avengers, X-Men, and X-Factor that he was responsible for destabilizing Scarlet Witch's abilities that led to her earlier issues, and then proceeded to use his upgraded magical abilities to take on the heroes before he was depowered and able to escape.
5 Kulan Gath - "An Age Undreamed Of"
While the powerful sorcerer known as Kulan Gath originally appeared as one of Conan the Barbarian's enemies in the Hyborian Age, he has returned to threaten the Marvel Universe a few times, though not many heroes remember it.
When he was released in the modern-day he altered the reality of Manhattan and its costumed heroes, transforming the X-Men, the New Mutants and the Avengers, leading to some of their deaths in battle with Kulan Gath before Magik and Doctor Strange were able to revert the timeline back to stop the sorcerer from awakening in the first place.
4 Sentry - Uncanny Avengers
When Robert Reynolds began to remember his erased past as the incredibly powerful hero known as Sentry, it brought the combined forces of the Avengers, the X-Men, the Fantastic Four and the Inhumans together just in case things turned violent.
Sentry eventually did succumb to his evil Void persona and was killed in battle with the Avengers, but he was brought back by the Apocalypse Twins as one of their Dark Horsemen, which required a combined team of Avengers and X-Men known as the Unity Squad to take them down.
3 Dark Phoenix - Avengers Vs. X-Men
Avengers vs. X-Men features the biggest showdown between Marvel's two powerful teams that fans have ever seen, and it was all caused by the approaching power of the Phoenix Force as it sought out a new host in Hope Summers.
While the teams were initially divided, once the Phoenix Force had arrived and corrupted Cyclops into Dark Phoenix, the remaining heroes were forced to work together and bring Hope Summers and Scarlet Witch together so they could stop Dark Phoenix and use its power to reignite the decimated mutant gene.
2 Thanos - Infinity Gauntlet
Thanos the Mad Titan recently proved to fans on the big screen how powerful of a threat he is when he took on the Avengers in the Marvel Cinematic Universe, though in The Infinity Gauntlet it took much more than the Avengers to stop him.
Unfortunately, the heroes didn't gather until after Thanos snapped away half of the universe with the Infinity Gauntlet, so only a small selection of X-Men and Avengers were gathered together to stop Thanos and they ultimately required the aid of cosmic heroes like Adam Warlock to save the universe.
1 Onslaught Vs. Marvel Universe
The psychic merger of Charles Xavier and Magneto created the powerful psionic being known as Onslaught, who stole the abilities of Franklin Richards and Nate Grey in an attempt to transform the world into a collective psionic consciousness like himself.
The Avengers and the X-Men teamed up alongside the Fantastic Four to take the fight to Onslaught, which ultimately resulted in the sacrifice of the majority of the non-mutant heroes, though they were unknowingly saved by the omega-level abilities of Franklin Richards.