Another matchup I’d love to see:
This is a tricky one to call, since they’re both absurdly powerful and nigh-invulnerable. But on consideration, I think I’d peg Sand-Man to win. The deciding factor is just that he’s more used to dealing with other super-powered beings like Spider-Man, while Clayface mostly goes up against the ‘merely’ well-trained and resourceful Batman and still consistently loses. Sand-Man may also regularly go down to Spider-Man, but he can certainly pose a tough challenge for him and Spidey’s a considerably more powerful opponent than Batman. Besides which, Flint Marko’s a career criminal and thug, so he’s probably been fighting all his life, while Basil Karlo’s an actor who would have had much less experience with violence before becoming a villain. Add in Sand-Man’s capacity to augment his own size and strength with sand he finds in his environment, something that I don’t believe Clayface can do, and I can easily see him coming out on top.
I picture Clayface kidnapping and imitating Marko’s daughter to try to manipulate him into some particular crime. Only, Karlo slips up at one point and Marko gets wise, resulting in him going full Papa Wolf on Clayface. This resulted in a savage battle of the criminal Earth elementals, finally culminating in Sand-Man absorbing an entire construction site or something in order to grow giant and splatter Clayface into a million pieces. Marko then carries his daughter off protectively in his arms, while behind him the individual drops of mud begin to slowly gather back together….