the question should be -- who could break it using physical strength alone. too many who could do it otherwise via magic or matter manipulation.
who could physically do it? hmm, i'll go out on a limb and say . . . no one. in theory, hulk COULD do it, but the scenario is pretty unlikely. hulk has tried before with the shield and failed. nefaria who appeared at least as strong as thor, couldn't do it.
i'm not sure it's the hardest substance in the MU (uru?? the destroyer armor??) but it is harder than pure adamantium and has survived an awful lot over the years . . .