These are both characters that I really love so I've got something to say about this.
The thing is that Goku and Superman are both sorta the same. They're both really good guys who, deep down, are even gooder guys. I don't know why people always pit them against each other because if they met, they'd probably spar a bit, then go eat a couple tables worth of food together. Superman does have the kryptonite weakness, but Goku is an honorable fighter and wouldn't use a trick like that. He always wants to test himself against stronger opponents, so if anything, Goku would probably only fight him at his strongest, which is to be in full exposure to a yellow sun. On the other hand, Goku needs to breathe and Superman doesn't, so Supes would have that option over Goku. But, being a just man, Supes wouldn't destroy a planet to defeat one innocent man. Tbh, I think they'd be pretty evenly matched. Akira Toriyama clearly thought of him as a sort of parallel character, being as Goku's own backstory is an almost word for word retelling of the destruction of Krypton. So, I think Goku and Superman, in addition to be equal in strength, would probably prefer sparring and forcing each other to get stronger than they would going into a death battle.
That said, I actually prefer Superman a little. Goku is awesome but for him, strength is an end. He wants to improve and be the best fighter, so he can fight better and fight stronger opponents to get stronger so he can fight stronger opponents etc. Superman uses his strength as a means to actively protecting people and making the world better. Goku primarilly sees strength as a goal to more strength, Superman sees it primarilly as a gift to be used to protect people.
