Galactus vs DC Earth (and allies)
I hope this hasn't been done before.
We've seen Galactus attack Marvel Earth a number of times now, usually he's defeated by a plot device, usually a Fantastic Four-related plot device but how would DC Earth fare against the Devourer of Worlds? Galactus is well-fed (so high showings but not Eternity-level full power) and has his five greatest Heralds with him. No UN though. And to make things a bit more interesting let's give Earth reinforcements in the form of five (non-Earth based) Green Lanterns (Killowog, Katma Tui, Raker, etc.) and the full resources of Darkseid and his planet Apokolips, because he doesn't want Galactus to do what's his job 😉 . This is full powered Darkseid, not an avatar.
First Scenario: One sided prep for the DC side (let's say a week's prep)
Second Scenario: Both sides get a week's prep
Third Scenario: No prep