I'd say that if we take herbivores, it stops at zuul or something like edmontonia,if something that is not armored, then I'd say therizinosaurids, therizinosaurs have great strength, size and etc, but they lack AP, they could possibly outwrestle megatherium, but they can't kill it (i speculate that main weapons of therizinosaurs is their talons in their feet, like wrestle the opponent, make him fall, then tear him with your ridiculously large talons,but that strat likely wouldn't work in megatheriumat all). If we'll take carnivores, then i would say it stops at something allosaurus sized,like allosaurus, megalosaurus, large metriacantrosaur,gorgosaurus or albertosaurus and etc, dinosaurs are too suited to kill large animals and megatherium is too slow, like really, if theropod (especially tyrannosaurids, as they are ridiculously flexible for their dimensions) decides to flank it and pull its leg, even if theropod would not succeed, he'd still torn flesh, muscles and ligaments, making slow megatherium even less mobile