Superman would be vulnerable to Thor because of the latter's magical nature/his hammer's magical nature, but would still have the advantage because he is so much faster. Also, Thor is tough, but not invulnerable. So Superman.
Superman, if we're talking about a fight that's not bound by morals and ethics. Thor if Superman gets all weak in the knees about why he's fighting, the damage the scuffle is making and with Lois shouting in his ear.
Thor might win if he stays close and wallops Superman with Mjolnir, maybe boosted by Odin or Norn stones or something but Clobbering Kal would strike hard, fast and from every angle. I really can't think of any Marvel hero that Superman can't defeat and that includes Doctor Strange, the Silver Surfer and the Hulk!
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Superman would be vulnerable to Thor because of the latter's magical nature/his hammer's magical nature, but would still have the advantage because he is so much faster. Also, Thor is tough, but not invulnerable. So Superman.
The question I always raise in such matters is: Which Superman?
Today's? I'd call it even.
The Golden Age Superman? Thor, easily.
The Silver Age Superman? Superman, no doubt.
The Bronze Age Superman? I think Thor could eke out the win.