The Marvel Cinematic Universe reached a temporary conclusion in Avengers: Endgame, with the defeat of Thanos after years of build-up. Ahead of the launch of MCU Phase 5, there has been much speculation about which villain will take Thanos' place -- and one suggestion is that hFantastic Four villain Galactus could open new realms of the Marvel Universe for the films to explore. However, that may be a let-down, seeing as Thanos has defeated Galactus in the comics -- multiple times.
Galactus' first defeat at the hands of Thanos came in the pages of Infinity Gauntlet, from Jim Starlin and George Pérez, as the universe attempted to put an end to the Mad Titan's control over all of reality. After the heroes of Earth failed to deter the power of the Infinity Gauntlet, cosmic beings including Galactus unleashed their power in the hopes of overwhelming Thanos.
Usually, the power of any of these cosmic beings would have been enough to destroy him, but the Infinity Gems allowed Thanos to easily imprison all of them. In this case, it was Thanos' recent power-up that gave him victory over the Devourer of Worlds.
Galactus was again defeated by Thanos at the onset of Annihilation, by Keith Giffen and Andrea Di Vito. Thanos convinced the Proemial Gods -- the first beings created in this iteration of the universe -- to launch a first attack on his target. After surviving the attack, a severely-weakened Galactus was captured by Thanos, who planned to use the being as a power source for Annihilus.
After Thanos abandoned his support of the Annihilation Wave, he attempted to free Galactus before being killed by Drax.
Cosmic Ghost Rider once detailed a future in which Thanos decapitates Galactus with a single energy blast. In all of their previous encounters, Thanos used his cunning and other cosmic weapons to give himself the advantage. However, in at least one future, he appears to gain enough power on his own to defeat the other cosmic giant without assistance.
In the current Marvel universe, although he's won against Galactus multiple times, Thanos' power isn't equal to his enemy's. In a Marvel Comics #1000 story by Ryan North and James Harren, a version of Thanos armed with the Infinity Gauntlet intends to destroy Earth. Unbeknownst to the planet's residents, Galactus defeats Thanos and takes his unconscious body, then leaves Earth to its own devices. He devours worlds, but he can also protect them -- and he's immensely powerful.
Galactus is a superior threat when engaged in direct conflict with Thanos. However, he may not be the right choice for the next MCU villain. He is a cosmic force that must satiate his hungers. For Galactus to truly replace Thanos, Marvel Studios would have to make the character significantly more evil and hellbent on destruction -- but then again, maybe a different kind of villain will be a breath of fresh air.