PBT and POM may be harder than ABS, which does mean that they're better, but I find ABS - at least if the legends are achieved by double-shot molding - quite adequate.
Another hard plastic - but, sadly, not suited either to double-shot molding or to a range of colors - is Bakelite. I think that's what the square keys on an old Flexowriter are made from...
A really hard material - and one which would protect the legends on the keys, thus eliminating the need for double-shot molding - would be some type of glass, if one were thinking of getting really fancy. But that has the drawback of having to wipe oil from one's fingers off the keys all the time, even if it would wipe off easily.
Marble or polished granite, I think, we will have to leave out of consideration.