I didn’t start this fight and I’m not going to end it, but here’s my contribution.
So who would win in a fight? Well, let’s look at what we’ve got. Both are the last(ish) members of their race and an alien protector on their adopted planet. They both fall for strong-willed Earth women. Both have died and come back. Both are considered fearsome fighters with strong codes of honor. Both have awful, awful movies that we will not speak about because they never happened.
Ok, that’s got nothing to do with their fighting ability, but I’m just saying there are parallels between them. (Also, I have no idea why they would fight instead of just becoming besties. They’re both so good natured that I’m not sure what they would fight about unless it was just for the joy of it, but that’s more Goku’s thing than Superman.)
So first things first, we need to do our research on the two. I’ve done some, but you should really take a look yourself.
As long as there’s a yellow sun and no kryptonite, Superman’s going to be pretty hard to beat. His skin can’t be cut, he’s superfast, can fly through space, has heat vision, and depending on the comic a couple other powers as well. His fights tend to be over pretty quick because no can fight him for more than a minute. He's just that powerful.
Goku can be hurt, but he’s also fast, can throw ki-blasts, make a world destroying spirit bomb (given enough time), and each time he gets knocked down Sayan Goku comes back stronger. Even if Superman killed Goku, he’d probably find a way to come back stronger than ever and the fight would continue. I mean, how many times has Goku died? He NEVER stays dead. Ever.
I’m not sure there would ever be a winner. The fight would just keep going and going, and the two would heal or revive between rounds. (Never ending fights are a staple of DBZ so Goku is used to them (Goku and Frieza fight for almost 20 episodes (3.5 hours!) in what might be the longest fight in anime history.))
But let’s be honest, do we really care who wins, or do we just wanna watch the fight?