Okay, so we have...two planet-eaters and one creature who essentially takes over every living being on a planet, regardless of how powerful they might be. Is that about the right assessment? I think so. Now then...
I actually have to go with Unicron.
I have reasons. The first is that any planet-destroyer who makes food of your world and has more than considerable enough power to do so automatically trumps gigantic forces of the much smaller in most cases. If we assume Starro has taken over...the DC universe...or enough of it, we still have to contend with the fact that BOTH Galactus and Unicron are so Big McLargeHuge that they can swat aside even the toughest of superheroes. So, that eliminates Starro and anything he can bring to the table.
The REAL fight is between two ancient-and-powerful planet-eating characters, Galactus and Unicron. Both of them have exceptional power. Galactus commands a pure power that can create a swathe of destruction across the galaxy more legendary than his eating habits. Unicron is not only energetically powerful, but makes use of physical toughness as well...which is something Galactus doesn't prefer. Actually, Unicron's known habits can be summed up thusly.,,
Now, as you know, Unicron's been defeated before. So has Galactus. Here's the thing that I see. Both of these guys are insanely strong, eating worlds and physically scarring planets with ease. The main difference? Galactus absolutely NEEDS to feed. He doesn't have TIME for a protracted battle while Unicron will go out of his way to PUNCH a planet because he was threatened. Galactus needs to maintain his power levels, or else something more powerful will emerge, a trouble in of itself. For the sake of the universe, for ALL universes, Galactus bows out in favor of Unicron.
