While Doctor Sivana uses his impressive intellect to cause trouble for Shazam, when it comes to physicality, Black Adam is the hero’s greatest opponent.
As far as a fight between the characters goes, yes, presumably Billy Batson would come out victorious in the end.
Black Adam derives his abilities from different mythological figures than Shazam, but his strength, speed, stamina, wisdom, power and courage on par with Shazam’s, if not sometimes higher.
That said, in real life, it’s reasonable to say with absolute certainty that it’d be Levi who’d be getting his candy ass kicked by Johnson.