I was going to do Galactus vs Unicron but I figured it's probably already been done, plus Rodimus already kicked Unicron's ass so I feel he's earned a shot at the Big G.
For the purpose of this fight Rodimus is equipped with the Prime Matrix while Galactus is fully nourished and has access to the Power Cosmic.
Galactus isn't allowed any backup from his heralds unless Rodimus gets his pick of Transformer allies.
Who walks away?