Galactus is one of the strongest beings in the
universe. He is possibly 3rd-4th in the universe (when fully fed).
Has total control over his own molecular structure. He
can change his size, and strength level to near infinite levels.
Has high telepathy. Also has high defenses against
telepathy. With this, he also has high telekinesis (able to move near
limitless sizes of beings/objects)
Can fire powerful bolts of cosmic energy. Levels of power vary, but the strength of them are near limitless. He can also create near unbreakable shields.
Can endow others with some of his cosmic power (like his Heralds)
Has Cosmic Awareness. This allows him to sense
disturbances across great distances.
Galactus' appearance is not his true appearance. He will appear to look like the species of whoever looks at him. If 100 different species looked at Galactus at the same time, they would each view him as his/her species. Galactus' true form is unknown (however, it is possible Galan's species was close to Human)
Great knowledge of the universe. Intelligence is far beyond mortal understanding.