Galactus vs. Unicron
What can I say? They are both huge, eat planets, create powerful servants and so forth. But which is "all that", as it were. Technically I was thinking of the comic version of Unicron, but feel free to think of his role in the movie.
Both come upon a tasty looking planet somewhere deep in space, and of course neither feels like saying "Oh no, you can have it." They would also probably have their servants with them.