Some of Marvel's most powerful characters are strong enough to shake the foundations of their entire universe. Despite the humble beginnings that many of them share, powerhouse characters like Doctor Strange and Doctor Doom have faced off against impossible odds with consistently impressive showings. But there's one member of the X-Men who may be more powerful than almost anyone in the entire Marvel Universe - and one of her variants proved it by going toe-to-toe with a Marvel's most overwhelming being.
In What If...? #27 (by Mary Jo Duffy, Jerry Bingham, John Stuart, Carl Garford, and Simek Chaing), the Jean Grey of a timeline where she didn't perish during the "Dark Phoenix Saga" ended up proving that she could fight Galactus and Terrax all on her own. It's a good reminder that the Phoenix Force (especially in the hands of an Omega-Level Mutant) is virtually unstoppable.
What If saw Jean Grey Beat Galactus
In What If...? #27, history diverged when the Phoenix Force (in the form of Jean Grey) took a blast meant for Cyclops during their trial by combat against the Shi'Ar Imperial Guard. The blast knocked her unconscious and left her incapable of turning into the Dark Phoenix, thus preventing her crisis of conscience and subsequent death. Captured by the alien empire, Jean was given an operation to prevent the use of her powers and was allowed to return to Earth otherwise safe and sound. The Phoenix Force lived on in this version of Jean Grey regardless, which eventually allowed her to hold her own against one of Marvel's greatest cosmic forces.
After learning of an alien world being targeted by Galactus, Jean is able to convince her teammates to lend their aid to the world. Although the X-Men can't defeat the World Devourer and his herald Terrax, the anguish of seeing Cyclops almost killed restores Jean's abilities. The Phoenix is then able to easily defeat Terrax one-on-one before facing off with Galactus. She's notably fast enough to dodge his attacks and powerful enough to injure him with her own abilities. When Jean eventually destroys his ship, Galactus decides to retreat to another world, warning Phoenix that someday her powers will cost her the very humanity she holds dear.
Jean Grey is Massively Powerful
Any character in any timeline being able to stand up to Galactus is a big deal, as the planet-consuming force of nature has long been cemented as one of Marvel's single most powerful entities. Galactus can consume worlds and defeat entire galactic armadas, with only the strongest characters capable of even being on his level. By contrast, the Phoenix of Earth-81727 is able to not just fight Galactus to a stand-still but force a retreat from the behemoth. While it's unclear whether this is the real Jean Grey of this timeline or the Phoenix Force masquerading as her, it's an impressive feat regardless. If it's the Phoenix itself, this is a good showcase of how powerful the cosmic firebird really is, and also suggests that it is more powerful than Galactus.
If it's Jean herself bolstered by the Phoenix power though, that might help explain how easily she was able to best Galactus and Terrax on her own. Jean is an Omega-Level Mutant, capable of slapping around villains like Nightmare who usually are powerful enough to even give Doctor Strange trouble. It's possible that even without the Phoenix Force, Jean could use her potentially limitless power to counter anything and everything Galactus can throw at her. But Jean having all that power might would also be a key weakness for the X-Men, as seen when she does eventually give into rage and sets off a chain reaction that dooms her entire universe. While the Jean Grey of Earth-81727 being capable of fighting Galactus is plenty impressive, it also serves as a brutal reminder of how easily Jean's power could be turned against the rest of the Marvel Universe - and yield disastrous consequences.