Shazam really never stressed himself with anything except versus Sivana, who was equally magically-superpowered.
Based off what I saw, and if Shazam doesn't goof off and actually goes full-powered fighting mode, he should at least get to Iron Man/Scarlet Witch at the least. Depending on just how durable he is (I don't remember him being hurt at all except for by, again, Sivana), Billy could get all the way to Thor if he uses his speed effectively against SW and Strange. One superpunch and they're both dead.
However, IMO the well runs dry when he goes up against primetime IW Thor, as Thor's strength and durability as well as his power output with Stormbreaker make it a one-sided stomp for Thor.
Marvel would bash him as well.
It's hard to gauge Shazam because he was such a sucky fighter. With the kind of powers he has, on paper he should be able to at least get to Dr. Strange. But with how he was depicted in the movie he probably stops at Hulk. He'll even struggle with Ironspider.
Also, Ironspider should be higher than Black Panther.