Carbon in the form of a diamond, of course, is the hardest element.
As far as metals go, modern high speed steels can get above a Rockwell C hardness of 70, and Osmium, as what might be the hardest pure metal, can be a hardness of 64 or more.
Most people would say Titanium, but that is incorrect because:
a) Pure titanium is very soft and not used for much of anything, but like nearly all metals titanium can be greatly improved with the right additives.
b) No titanium alloy can be hardened much above a Rockwell C of 50, and at that hardness Titanium is very brittle. Most titanium alloys will be used at 40 or so, so they can bend and stretch as needed for much greater impact strength.
I hope this helps...