I’ve always wondered. I’m well aware of their fights and the fact that they’re both incredibly strong beings and I would have to naturally assume Superman due to his just insane feats of strength but the fact that at first Superman thought Shazam was kryptonian and that he can go toe-to-toe with Superman makes me wonder…
Always thought Superman had two vulnerabilities: kryptonite and magic. As Shazam is a being of magic he should have the upper hand. That being said Superman has a lot of experience that young Billy B doesn’t have so I’d throw the fight in Superman’s direction. (Experience beats magic I guess!)