I've compiled this small one panel comic version of Goku and Superman battling against each other, since I'm a nut over cross-over fights. Mono vs Mono, Alien vs Alien, Saiyan vs Kryptonian, Kamehameha vs Heat beam.
This is for a collab of a sprite comic an author created between Goku and Superman battling.
Because fights like this will never have a definite answer upon who will win (Unless both the creators of Superman and Goku sat down together and created a comic of the outcome) but I'd like to think that it would be roughly evenly matched.
Superman is shown to be able to move planets, heat up a planet, super speed, strength, heat beams, freeze breathe, Super senses, high stamina, ability to breathe in space, ability to become stronger after absorbing solar energy, and skilled in the ancient arts of fighting (however, for some reason the creators only introduce Superman's fighting abilities whenever he loses his powers).
This leads me to believe after looking at various comics and discriptions of Superman, that the Man of Steel's powers were never fully shown to the max because, like many comics, it would make the fights too easy, and ever since Superheroes got together in comics, they had to tone down Superman to give the spot-light to other super-heros. That, and because it was becoming increasingly difficult for the writers to come up with different stories for Superman while he was powerful.
Though this keeps him from using his powers to the max, Superman has been shown to have large willpower to battle against various antagonists, such as Doomsday, to the bitter end. Because Superman battles the same foes that becomes stronger and smarter, this forces him to have to be even more resourceful and stronger than his opponent.
Goku is known to be able to destroy a planet, Teleport from one Universe to the next (not all in a second, but in about a couple of hours I think), Super strength and Speed, Stamina, harness and use the abilities of Ki, the ability to become stronger after every fight (though it takes a long while) and the ability to absorb energy from lifeforms around him.
Because every antagonist Goku faces have similar abilities such as himself, it gives the Saiyan plently of oppurtunities to dig deep into himself and fight with every ounce of himself, becoming stronger and Actually Achieving the ability to go Super-Saiyan and up.