I did some checking, and I came up with a few possibilities.
The first one I found is called Inertron, which is described on Wikipedia as the "the hardest, densest substance in the DC universe."
Another choice would be "Supermanium", which is apparently the substance used to craft Superman's Supermobile aircraft. It apparently is vulnerable only to one of Superman's power.
a metal so strong that "only Superman's heat vision can soften the substance, and only his super-strength is mighty enough to mold it!"
Finally, there is Nth metal, a magical metal used by Hawkgirl and Hawkman. It's not clear how physically durable/resistant the metal is, but it does imbue the wearer with tremendous powers. It seems reasonable to assume that it is an extremely strong substance as well.