A Cosmic Cube gives the possessor of it anything they want, basically it grants wishes. The first time Doom had a Cube was when the Beyonder and the Molecule Man joined to become the Cosmic Cube they where suppose to be, doom wished for his memory back then Kubik and the girl he was with banished doom and the FF back to earth.
The Red Skull could have wished the power of Galacus or anything he wanted, if he lost to Captain America while the Skull has a Cube then he just doesn't have much of an imagination.
I would have to say Galactus in this fight. If I'm not mistaken the Cube grows to become a beyonder and they are between Eternity and Death as is Galactus. but Galactus is more then just his power, he is part of the Universe.