Superman would have no chance against Dr. Doomsday, who has the power of Doomsday coupled with the intelligence and the character of Dr. Doom. So to make things more interesting, take Superman when he was the herald of Galactus, who had made him "his most powerful herald", against Dr. Doomsday when he stole the power of the Silver Racer (fusion of the DC Black Racer and Marvel's Silver Surfer).
Doctor doomsday can use magic as a Doctor Doom but Superman does not have that particular weakness for this fight. No Kryptonite weakness or a red sun either. Galactus abolishes his specific weaknesses.
No preparation, no outside intervention. Space vacuum as a battlefield.