Though it might be surprising to those who aren't as familiar with comic books, Marvel's most undefeatable hero is easily Squirrel Girl. Originally debuting in an issue of Iron Man where she helped defeat Doctor Doom, the character's running joke became that she was virtually unbeatable. This has led her to come out on top against multiversal threats such as Galactus, Thanos, and Mephisto. Despite this though, her greatest achievement of all saw her defeat every single hero in the Marvel Universe.
In 2016's aptly titled Unbeatable Squirrel Girl Beats Up the Marvel Universe from Ryan North and Erica Henderson, Marvel's heroes are forced to confront an eventuality they never planned for; what if Squirrel Girl went bad. As one of the purest and noblest of Marvel's heroes, this might seem unthinkable, however, an unfortunate series of events make it a grim reality. When Iron Man invites Squirrel Girl to bring her squirrels to test out a new gadget he confiscated from the High Evolutionary, the two heroes are forced to face off against the High-Evolutionary's goons. Squirrel Girl is eventually knocked into the pod-like device, which is when two Squirrel Girls emerge. Though Doreen and her clone, dubbed Allene, initially seem like fast friends, it's quickly evident that something went wrong with the cloning process.
Allene decides that squirrels would make better stewards of the planet than humans, and thusly uses the High Evolutionary's pod to clone an army of squirrels. Using Squirrels, she steals Iron Man's suit and Squirrel Girl's copy of Deadpool's Marvel trading cards, which helpfully explain every single hero and villain's weaknesses. Allene then blows up Stark Tower and tricks the Avengers into thinking she's the real Squirrel Girl, only to electrocute them and steal all of their gear. She goes on to steal Magneto's helmet, Doctor Octopus' arms, Doctor Strange's Eye of Agamoto, and the Silver Surfer's surfboard. With all of that powerful equipment, she easily demolishes every single Marvel hero and villain one at a time.
While her rampage is eventually stopped, the only person that could stop her was, appropriately enough, the Mjolnir enhanced real Squirrel Girl and the rest of Marvel's wide roster of heroes. Though readers only see a handful of heroes in the epic final battle, the caption at the bottom of the page informs readers that every Marvel hero canonically takes place in the fight regardless of whether they're seen or not.
Aside from the obvious humor, one of the things that makes the situation so fascinating is that this story is entirely canon. While Marvel has had a few stories about their heroes slaughtering the rest of the Marvel Universe, such as Deadpool Kill the Marvel Universe, this is the only hero rampage that actually took place entirely in the main 616 universe. It's telling of Squirrel Girl's character that even when she battles the entire Marvel Universe, she never actually kills anyone. Even at her absolute worst, Squirrel Girl is still a truer hero than someone like the Punisher. Still, given what she's capable of, Marvel's heroes should start taking Squirrel Girl much more seriously.