1. "The Silver Surfer" #4: This is the most iconic defeat. Galactus was using the Silver Surfer as his herald to devour planets. The Surfer, having developed a conscience, rebelled and appealed to the Fantastic Four for help. They used the cosmic energy of the Surfer's board to overload Galactus' energy-absorbing device. This weakened him enough for the Surfer to redirect the attack, sending Galactus hurtling into the sun and forcing him to retreat.
2. "Fantastic Four" #244-247: Here, Galactus was being manipulated by the villain Abraxas, who was trying to destroy the universe. The Fantastic Four managed to separate Galactus from Abraxas' influence, freeing Galactus and allowing him to fight back against his manipulator. The Fantastic Four then convinced Galactus to work with them to defeat Abraxas.
3. "Fantastic Four" #570-574: This storyline, "The End is Nigh", involved Galactus turning against the heroes who had saved him from Abraxas, seeking to devour Earth. The Fantastic Four, alongside other heroes, launched a massive attack. They ultimately defeated him by using a weapon called the "Ultimate Nullifier", a device capable of destroying anything. This victory, however, came at the cost of the universe, as the Nullifier destroyed the universe and recreated it as a new one.
Important Note: Galactus is a cosmic entity of immense power, and his defeats are often temporary. He has returned countless times, and his power is constantly evolving. The Fantastic Four usually have to utilize a combination of cunning, ingenuity, and sheer luck to defeat him.