Marvel Comics loves its crossover series and has scheduled them usually once a year for the last two decades. The first major crossover in Secret Wars and the huge Contest of Champions saw Grandmaster and Collector make a bet on which heroes were most powerful, and they've become a tradition ever since.
There were smaller crossovers before this, including the Kree-Skrull War in the 1970s, but nothing like what would come a few decades into the future. In Marvel, the crossover series have always been world-altering, with the recent King in Black and Heroes Reborn threatening to change everything that fans know about their favorite heroes. However, some series are not as widely accepted as others, and voters on Ranker have their own ideas for the best of the best.
Siege (2010)
Marvel's Siege was a 2010 crossover event that saw the Asgardians attacked and at war with a cabal led by Norman Osborn, who himself had been working for the U.S. Government since the end of the Secret Invasion crossover event. At this time, Asgard was on Earth as all the gods had reincarnated and lived in their city, which floated above the ground in Oklahoma, making them their own sovereign nation.
The attack came when Loki betrayed his brethren and helped Norman get the information he needed to attack. The main series was only four issues, but there were crossovers in the pages of the various Avengers issues as well as New Mutants, Thor, and Spider-Man.
Avengers Disassembled (2004)
Avengers Disassembled did the unthinkable. It ended the Avengers. This crossover series started when Wasp was lying out at the pool with Wanda Maximoff and made an offhand comment about Wanda having children once. That was a memory removed from her mind, but the mention reminded her that she had her children taken away.
Scarlet Witch snapped and fabricated a Kree attack on Earth that ended in tragedy. By the end of the event, Ant-Man, Hawkeye, Vision, Agatha Harkness, and Jack of Hearts were all dead and Doctor Strange had to shut down Wanda's mind to stop her reign of terror.
Infinity (2013)
There were three different Infinity Gauntlet storylines for Marvel involving the fight for the Infinity Stones. In 2013, the entire storyline returned, likely as a way for Marvel to help promote their upcoming Avengers movies with Thanos and the Infinity Gantlet.
In this 2013 storyline, the Avengers left Earth to battle some villains, and Thanos used the opportunity to attack the planet with his Black Order since Earth's Mightiest Heroes were gone.
Avengers Vs X-Men (2012)
In 2012, it was time for the Earth's Mightiest Heroes to battle the Earth's strongest mutants. Avengers Vs. X-Men started out with the news that the Phoenix Force was headed back toward Earth. When everyone realized that Hope Summers was the likely target for the new Phoenix host, the two teams had different views on how to deal with this.
The Avengers wanted to lock away Hope and try to keep the Phoenix from getting to her. Wolverine was even ready to kill Hope to stop it, siding with the Avengers. However, the X-Men decided it might be better to allow it to happen, and the war began. The Avengers succeeded, but the Phoenix took on five hosts and the war grew out of control until Professor X was dead and Cyclops was in prison as a martyr to the mutant cause.
Spider-Verse (2014)
Through the 2000s, Spider-Man comics started to build on the idea of the Spider-Verse, with different versions of the Wall-Crawler active. In 2014, it all came to a head with Spider-Verse. This crossover event started with the reemergence of Morlun and his family, the Inheritors, as they determined to kill every Spider-Totem in the Multiverse.
They actually did kill several of them before a time-displaced Superior Spider-Man created a team to fight back. This included dozens of Spider-Totem heroes banding together to stop the seemingly unbeatable immortal killers.
World War Hulk (2007)
One of the most popular Marvel storylines of all time was Planet Hulk, where the Illuminati banished Hulk from Earth and he became a hero on a new planet. However, when an explosion killed his pregnant wife, he set off for Earth to seek revenge against the heroes who betrayed him.
The crossover was World War Hulk, and this was one of the most powerful versions of Hulk in existence. With his warrior friends from Sakaar, he successfully exacted revenge on the Illuminati before it finally ended when Hulk stopped fighting and allowed himself to be captured.
Secret Invasion (2008)
Secret Invasion changed a lot when it came to the Marvel Universe. In this Marvel event series, the heroes of Earth learned that not all of them were human and that the Skrulls had been capturing heroes for a long time and replacing them with the plan to conquer the planet.
This led to the Skrull Queen leading an attack on Earth, with the heroes not knowing who they could trust. This brought back heroes fans thought had died and also ended with Norman Osborn killing the queen and saving the planet, becoming a hero that would soon use his fame to take over S.H.I.E.L.D. and lead the Marvel Universe to Dark Reign.
House Of M (2005)
In 2005, the Scarlet Witch changed the entire world to her design. After Avengers Disassembled, Wanda had her mind shut down by Doctor Strange and was taken by Magneto so he could care for her. However, while they had her in their care, she cast a spell that changed the world into one where mutants ruled.
The twist with House of M was that everyone had a better life than they did in the real world. However, Wolverine could sense that things were not right and set out to fix things. When he succeeded and it was all fixed, Scarlet Witch depowered almost every mutant on the planet.
Secret Wars (1984)
The first crossover that put almost every character in Marvel Comics into the same story was Secret Wars. This was a huge event for readers at the time, as all the main comics in one month ended with all the heroes disappearing and then reappearing the next month with some major changes.
That led to the 12-issue Secret Wars series to show what happened to change the comics. The heroes and villains were taken to Battleworld by The Beyonder and were forced to fight, with the winning team getting anything they wanted. By the end, Spider-Man had a symbiote, She-Hulk was a member of the Fantastic Four, and more.
The Infinity Gauntlet (1991)
The Marvel crossover event that ranked as the best in history according to Ranker was The Infinity Gauntlet. This was similar to the storyline in the MCU movies, but with a few changes that helps it stand out. In the comic book crossover storyline, Thanos was in love with the character of Death and wanted to win her adoration.
He gained the Infinity Gems and then wiped out half of humanity. However, he ended up facing powerful heroes like Silver Surfer and Adam Warlock as they helped reset the universe. The crossover was so successful that two more Marvel crossovers followed with Thanos in The Infinity War in 1992 and The Infinity Crusade in 1993.