Green Lantern is commonly seen as one of the most powerful members of the Justice League, and for very good reason. No matter who is wearing the ring, they are consistently a powerhouse of willpower, providing insane lantern constructs and a large helping hand with the team. Whether it's a small issue in Coast City or a galactic invasion, there are not many villains who can stand up to Green Lantern's light throughout the universe.
The same could be said if the Corps traveled to the Marvel Universe. However, even Marvel's cosmos has some insane villains, so there are a few exceptions who could send the ring searching for a new host.
10 Could Defeat: The Sentinels
The Sentinels are a group of powerful, giant robots that were created to hunt down and eliminate mutants. This obviously puts them at odds with the X-Men on numerous occasions, throughout the comics and in live-action adaptations like Days of Future Past.
First of all, none of the Green Lanterns are naturally powerful, as all of their abilities come from their ring, so it's highly unlikely the Sentinels would target them at all. If they did, however, any of the Green Lanterns should be able to defeat them easily.
9 Couldn't Defeat: The Void
Over the years, Marvel has desperately tried to create its own version of Superman. We've seen Hyperion, Gladiator, Blue Marvel, Virtue, and the list goes on. But the most well known and probably most successful has been The Sentry. Robert Reynolds is one of Marvel's most powerful heroes, but there is a dark side to all that power, and it has a name: The Void.
It's a destructive counterforce that is constantly at odds with Reynolds, and when it overtakes the body, it's a really bad day for Earth's heroes. GL would definitely need the Justice League as backup, because alone he definitely doesn't stand a chance.
8 Could Defeat: Abomination
Abomination is much more than just evil Hulk. Emil Blonsky was a former KGB agent that willingly doused himself in gamma radiation to become just as strong as The Hulk. Unlike Hulk, however, he retains his human intelligence, and his transformation into the creature was permanent.
While he is a worthy foe of one green hero, Green Lantern has taken on villains far more powerful, including Arkillo, who is similar to Abomination in size and strength, with a Sinestro Corps ring for good measure.
7 Couldn't Defeat: Doctor Doom
Doctor Doom is arguably the most iconic supervillain of all time. The former classmate of Reed Richards, Doom has mastered the arts of magic and technology to become the most powerful foe of the Fantastic Four, and one of the top masterminds in the Marvel Universe.
He even eventually managed to reach godhood during the events of Secret Wars and is a constant antagonist to Marvel's greatest heroes. He could take on the Justice League, much less Green Lantern by himself.
6 Could Defeat: The Sinister Six
When these guys team up, you know you are in for quite the Spider-Man story. Consisting of his most powerful foes, the Sinister Six are a supervillain team that could only be rivaled by Flash's Rogues. Their team mainly consists of Green Goblin, Doctor Octopus, Electro, Sandman, Mysterio, The Rhino, Vulture, Chameleon, Kraven, or Scorpion, depending on who is writing them at the time.
While they are an incredible group that comes close to beating Spider-Man multiple times, the Webhead always manages to gain the upper hand. Since Green Lantern is much more powerful than Spider-Man, it's safe to assume he would be able to defeat them as well.
5 Couldn't Defeat: Dormammu
No matter how many times Green Lantern could come to bargain, it's hard to imagine he'd be able to defeat Dormammu. Dormammu is one of the greatest forces of evil throughout the Marvel Universe. He has an infinite number of extra-dimensional powers as a result of being born in the dark dimension.
He is easily able to go against powerful magic users such as Doctor Strange, Loki, Mordo, and Clea. Maybe GL should bring Doctor Fate as a backup for this one.
4 Could Defeat: Venom
Venom is typically seen as an anti-hero these days, but at one time he was Spider-Man's most terrifying villain. Venom was created when Peter Parker discarded his alien symbiote suit and it was picked up by Eddie Brock, giving him Spider-Man's powers and a variety of other skills.
Whether its Eddie, Flash, or even Mac Gargan wearing the symbiote, its weaknesses to sound and fire should be easy enough for Green Lantern to expose and defeat another alien that threatens innocent lives.
3 Couldn't Defeat: Galactus
Galactus is typically seen as the end all, be all to Marvel's cosmic villains, and for good reason. He is powered by feeding off of entire worlds of civilization, and it's no doubt he would eventually find his way to Sector 2814 to polish off his appetite.
Green Lantern would probably need the entire Corps watching his back if he wanted to take on the colossal titan, considering Galactus has often had to fight against the combined efforts of the Marvel heroes, and it still often isn't a fair fight. Could Green Lantern making a construct of the Ultimate Nullifier be enough to replace the original? Probably not.
2 Could Defeat: Magneto
Magneto is definitely one of the most powerful mutants to cross and join the X-Men in the team's history. Fighting for the mutant cause, his views are definitely more radical than Professor X's, as he believes humanity is an inferior species and should bow down to mutants rather than accept and cooperate with them.
He can occasionally be seen teaming up with Professor X, but he almost always goes back to his ways with his Brotherhood of Mutants. The Master of Magnetism is definitely more powerful than DC's version, Doctor Polaris, but there can't be much Magneto can throw at Green Lantern that he probably hasn't seen before.
1 Couldn't Defeat: Thanos
Of course, any list describing powerful Marvel cosmic characters would simply not be complete without Thanos. Through hundreds of comics and his role in the MCU, Thanos has skyrocketed his popularity to rival Doctor Doom and Magneto. He's obviously insanely powerful, having defeated the likes of Odin and Galactus before.
With the Infinity Gauntlet in his hand, he's even more powerful, and Green Lantern stands no chance facing off against him, unless he has the White Lantern at his side. Then that would be one hell of a battle.