DISCLAIMER: I do not own Superman, the Justice League, Dragonball Z, Dragonball GT or Son Goku.
Superman flew across the planet, having heard something disturbing. He arrived at the place where the explosion had taken place, and was just in time to see a man with spiky dark hair, dressed in an orange martial arts costume, fling a ball of blue Ki energy straight through his enemy, a skinny blue alien in a strange space suit.
Superman approached as the skinny alien hit the concrete, dead.
"Who are you? What are you doing here?"
The spiky haired man turned to face Superman, blinking in confusion.
"I'm Goku. I'm just getting rid of someone who wanted to kill some people. He was just some criminal who wanted to kill people."
Superman moved straight up to Goku, glaring at him.
"You just did some serious damage. Who knows who you injured?"
Goku nodded. "I know. I tried to take the fight away from the city, but he would've done more damage on the way out. He was weak, so I thought it would be better to just destroy him quickly."
Superman nodded. "I understand. And I don't expect everyone to adopt my no killing policy. You did the right thing."
Superman and Goku stood in the Watchtower, looking down at the Earth.
Goku had explained his origins to Superman, and Superman had in turn explained his.
"A Saiyan, eh? I've heard some vague accounts of them. Weren't they wiped out by some madman named Frieza?"
Goku nodded. "They were. Me and Vegeta are the only two pure Saiyans left. My sons Gohan and Goten, and Vegeta's son Trunks, are half Saiyan, but also half human."
Superman nodded. "And the woman you call 18 is an android?"
Goku nodded. "She used to be human, but her and her brother were captured and turned into androids."
"That would make them cyborgs, not androids."
Lex added the finishing touches to his machine. The machine would replicate the effects of various forms of Kryptonite, and would in theory drive Superman insane.
Superman stood over a man who had stolen another man's wallet.
"I'm guessing that isn't yours."
Superman punched the man unconscious, before flying over to the elderly man whose wallet had been stolen from him.
"Yours, I believe."
Superman returned the man's property, and flew away. As he was flying through the air, he was struck by a terrible feeling. His vision blurred, and when it cleared, he was different.
Superman remembered where he was going, but he didn't remember why.
The Justice League were his former allies, and now his target.
Superman hovered over Wonder Woman. She turned to face him.
"Clark. What's wrong?"
Superman charged at her, and she was not fast enough to defend herself. His fist connected with her face, driving her straight through the ground. He felt her bones break slightly under the force of his blow. He stood over her, ready to finish her off. Wonder Woman started to say something, but fell unconscious before she could. Just as Superman was about to flatten Wonder Woman's skull, someone gripped his wrist. He turned to see Goku.
Superman swung his other fist at Goku, striking him in the face. Goku was sent flying straight down the block, smashing through several buildings in the process. A badly bruised Goku stood, bleeding from some very deep cuts from the metal framework of the buildings. With an explosion of yellow energy, Goku powered up to Super Saiyan, preparing to fight Superman.
Super Saiyan Goku used his Instant Transmission to reach Superman in an instant. The two then started raining blows on each other. Superman was faster and stronger than Goku, but Goku was more skilled. Regardless, Superman was winning. Superman punched Goku in the jaw, launching him into the air, and following him up. Goku fired a Kamehameha at Superman, but Superman responded with his heat vision, and each fighter poured huge amounts of power into the respective blasts. Each one had limited energy, but Goku realised that his energy would fail before Superman's did. Goku used Instant Transmission to reach a safe distance, and powered up to Super Saiyan 2.
Super Saiyan 2 Goku attacked Superman with a Ki blast, but it still had little effect. Superman flew into Goku, and the two started exchanging lightning fast blows. Goku kicked Superman in the face, staggering him slightly, but Superman responded by slamming his fist into Goku's face, hitting with such force that Goku was launched back several hundred metres, his face bleeding. Realising he was still outmatched, Goku powered up to Super Saiyan 3.
Super Saiyan 3 Goku flew to Superman, firing a Ki blast at him. Superman took the Ki blast to the chest, grunting with pain, but not particularly effected. As Goku prepared to attack with his fists, Superman flew into Goku, collecting him. As Superman carried Goku through the air, Goku powered up to Super Saiyan 4. As Goku was about to attack, he was hit by Superman in the face, and fell from the sky, his jaw feeling broken. Superman caught up to him, punching him several more times, severely damaging him.
Super Saiyan 4 Goku fired a Ki blast into Superman's face at close range, launching Superman through the air. Superman flew back and shoved Goku back, and Goku hovered in front of Superman, preparing to use his Dragon Fist technique, which would never even cross his mind if he were in a more stable form. Super Saiyan 3 and 4 made Goku noticeably more violent than usual. Goku flew at Superman, slamming his Dragon Fist into Superman's chest. Goku's fist penetrated Superman's chest, and the Ki dragon that was created started to wrap around Superman, but Superman did not disintegrate from the damage, though it did hurt him very badly, causing him to audibly cry out.
Superman punched Goku in the face, breaking the Ki dragon that surrounded him, and launching Goku back several hundred metres.
Goku struggled to his feet, just in time to have Superman grip his neck, picking him up. Goku kicked Superman, but Superman showed no sign that he felt it. Still gripping Goku's neck, Superman slammed his fist into Goku's face, causing considerable damage. His energy spent, Goku phased back to his base form, waiting for Superman to finish him. As Superman raised his fist to deliver a blow that he knew would shatter Goku's head, He blinked, dropping Goku onto the ground.
"Goku, are you okay?"
Lex Luthor stood in front of his machine, running several counters over it to verify that it was putting out the radiation he wanted it to. Convinced as he was that Superman was insane from the machine, he was very surprised when Goku and Superman shattered the wall behind him, stepping into the room. Lex put on the suit that replicated some of Superman's solar powers, but Superman pulled Luthor out of it, breaking the suit in the process. As Superman subdued Luthor, Goku put his fist through the machine that had driven Superman temporarily insane, stopping it instantly.
Superman stepped up next to Goku at the Watchtower, where Goku leaned on the glass, looking down at the Earth.
"Sorry if I hurt you, Goku."
Goku shook his head. "Ah, don't worry about it. It bothers me a bit that I lost to you, though."
Superman leaned against the glass with Goku. "I guess that means you want a rematch?"
Goku smiled. "Yeah. Not now, though. Some other time. When I'm better prepared."
Superman shrugged. "You don't need to work on your skills. I'm the first to admit that my skill's average at best. Thing is, I don't really need skill. I'm Superman."
Reasons for this outcome: 1. Superman is clearly stronger than Goku, even though the difference in speed is made up for by Instant Transmission. 2. While he can still be exhausted if he overexerts himself, in particular his heat vision, Superman continuously draws energy from the sun, whereas Goku wears down and stays worn down. 3. Goku is physically vulnerable to all damage, having been shown to be severely burned by lava, which would make him vulnerable to Superman's heat vision, and will die if physically broken, which Superman's blows would eventually do to him. The only way he could withstand any hits from Superman is if he is in his higher forms, and damage will eventually tire him out and revert him to his base form.