Before I dig into this, let me start with full disclosure. I am fans of both Goku and Superman. If you don't believe me, check out my top 5. I'm well versed in both heroes. But the Goku v Superman debate just won't die. ScrewAttack actually felt the need to do a rematch to try and settle it. I won't spoil it for you if you haven't seen it. I embedded the video below. Aaron Schoenke is mulling over a Goku V Superman fight on Super Power Beat Down, and the outcome of that will likely be a Goku win, which Goku fans will then trumpet as definitive "proof" that Goku is tougher. I kind of hope this happens actually. As much as I'm sick of hearing the same erroneous crap from Goku fans all the time, I could at least then point out to them that by heralding that win as proof, they would have reduced Goku to a kind of Tinkerbell hero. If we believe in him enough, he can beat anyone! It's funny.
This debate has its origins in the similarities between the two characters, I think. They're both the "lone" survivors of a dead alien race; they live on earth with extraordinary powers, and they're both good guys who care about other people. In some ways Goku is the Japanese Superman, especially since he's (at least for me) the measuring stick that lots of other anime heroes are compared to, not unlike how Superman is the prototype for all other western superheroes.
On one level, it makes sense that Goku fans want to pit Goku against everyone, because that's the whole plot of Dragon Ball Z (and still is). Goku wants to prove himself against the toughest guys he can find, and he (almost) always wins. So Goku fans look at the mighty Superman and think: Ah! The ultimate challenge. Goku has beaten everyone else, surely he can beat Superman! They're basically the same anyway.
Except there's a fundamental difference here that people are overlooking, and it's a cultural one. Dragon Ball Z comes from a culture that placed a lot of emphasis on one-on-one competition between warriors. Superman's creators are less interested in how he measures up to individual warriors, and prefer instead to examine his personal struggles as a character as well as greater societal issues. When it comes to beating the bad guys, Superman usually wins through cunning and strategy, not increasing his power level. The latter method is a more self-focused approach. Goku cares about saving his friends, sure, but he cares even more about proving he's the best martial artist. Superman cares most about protecting the innocent and doesn't give a damn who is stronger or tougher than him. That's the difference. The reason Goku could never beat Superman has nothing to do with power levels and everything to do with motivation. Goku fights for himself, and he's driven by pride that prevents him from accepting help from anyone other than himself. Superman never fights for himself, so he will always employ all of his allies and resources to save the day. Thus, Goku can only beat enemies if he surpasses them, but Superman doesn't need to surpass a foe to beat him.
I would go so far as to say that Goku (not just Goku but everyone in DBZ) is willing to put the world at undue risk in order to have his one-on-one pro-wrestling match with a bad guy. Okay, okay, that might be mischaracterizing it a bit. It's more like a Samurai duel, but still. Remember the final battle against Perfect Cell? Everyone knew Goku wasn't as strong as Cell, and everyone knew that Cell was basically holding the world hostage to have a "tournament" in which he would systematically torment and kill the world's strongest fighters for his own twisted amusement. The world was officially in peril. So what do the world's strongest fighters do? Do they selflessly put aside their desires to win the championship belt and work together to bring down a foe bigger than all of them? Nope. Goku says "Stand back, I'll handle this," and then after failing, he steps back and says "Lol, jk. Gohan, you handle this."
Some of you are thinking "you're effing stupid, how could they work together if none of them were strong enough to hurt Cell?" Not true. Neither Goku nor Vegeta were as strong as Perfect Cell but both of them managed to blow away half of Cell's body. So clearly, you don't have to be stronger than someone to deal a fatal blow in the DBZ universe. If not for Cell's regeneration powers, Vegeta would have ended the saga on his own, and he isn't even as strong as Goku! So imagine what they could have done with teamwork. But that comes back to the difference between Goku and Superman. Goku won't do anything to compromise his pride as a warrior. Vegeta explains that Goku would refuse to even use a senzu bean to carry him through the fight with Cell. Thwarting Cell wasn't as important as defeating Cell. See the distinction? If he can't win the belt (I'm using "belt" as a metaphor for being the toughest), on his own power, then he doesn't want it. Earth be damned. It might seem honorable, and as long as the belt is the only thing at stake, it is. But when the whole freaking world is at stake, it seems kind of selfish and reckless.
Superman does whatever has to be done to beat enemies who threaten earth, short of murdering them. His pride is never a factor. He's not interested in proving he's the greatest warrior. In fact, none of his friends are, except maybe Hal Jordan, but that's because he's an insecure showboat. When Darkseid attacks earth in Justice League, Superman doesn't tell his teammates to stand back while he tests himself. That would be foolish. They all work together to overpower the titan because Darkseid is bad news, and Superman doesn’t mess around with bad news.
These imaginary contests between Superman and Goku all take place outside of a narrative. It's literally just a comparison of stats and numbers, and when it comes down to that, yeah, Superman wins by a mile. If there were an actual story behind the conflict, then Superman probably wouldn't even fight Goku. Superman's humility and Goku's pride would be a bad mix. He wouldn't care who the strongest is. Goku would have to assault Superman to goad him into fighting, but that wouldn't be the contest Goku would want; it would be self-defense. Goku wouldn't push it that far because he's not a bad guy. Superman would probably just say "you can be the strongest. That's fine with me. Go ahead." But if Goku ever became a threat to planet earth, you better believe that Superman will take him down. It doesn't matter how powerful Goku is, because Superman doesn't limit himself to his own powers and skills the way Goku does. Superman's pride isn't a factor, so he'll do whatever it takes to win.
Ever read Superman's battle against H'el? He was another general Zod who came after Superman from Krypton, except he had weird reality-warping powers (of which no explanation was given), so he was way more powerful than Superman. He could literally just blink Superman away. So what does Superman do? Does he train hard to get to the next power level? No. He mobilizes the freaking Justice League. Ultimately, it wasn't even Superman who defeated H'el. It was Supergirl, who betrayed him and stabbed him with kryptonite. That's sort of how western superhero stories get resolved. Goku wins all of his fights by being able to punch, kick and blast harder and faster than his opponent. Those stories are all about the honor of the duel. Vegeta once said Goku was a "genius" at fighting, but he never does anything different. All he can do is just break through to the next power level. That's all he ever does. It's two-dimensional thinking (and it gets boring after a while). Superman and other western superheroes win by thinking outside of the box. They imprison their foes, trick them, turn their own powers or weapons against them. They win by cunning and resourcefulness.
Imagine if Goku had worked together with his friends to defeat Cell. What if they reasoned that since Cell had all of their powers, he must therefore have all of their weaknesses also? Perhaps they might have tried to exploit that by, say, exposing Cell to the heart disease that killed Goku. And no, I don't think Piccolo's powers would have saved Cell from that because Piccolo's powers seem only to restore damaged or lost tissue. There's no evidence that he also has defenses against pathogens. But I digress. No, it would never occur to the DBZ guys to cooperate because that's not acceptable in their culture. They would rather die and let the world be destroyed than compromise their pride. For Superman and his friends, the only dishonor would be failure to save the day. Murder would also be a dishonor for him, but I think he'd rather accept that shame as the lesser of two evils.
Hence, if Goku were to pursue Superman for a duel of honor, Superman would most likely tell him to buzz off. If Goku ever wanted a fight with Superman, he'd have to become a threat to the safety of every person on earth. And even if Goku could somehow achieve power level infinity (probably rainbow colored hair? Or maybe polkadots?), it wouldn't matter. Superman would figure out a way to win. It wouldn't be by getting stronger. It would be with cunning, teamwork, and resourcefulness. Goku's pride to win by himself under his own power will always be his greatest handicap. Superman will never hesitate to employ all of his resources and allies to win because unlike Goku, he never fights for himself.