Which material is the most resistant to the penetration of an x-ray?

Really, any material can deflect x-rays. It all depends on the density of the object in question. Lead used to be used when doctors did x-rays, but the sheets of lead were very heavy. If the lead is thick enough, it will actually absorb the x-ray. So, as a short answer, any material is resistant. It all depends on the density of the object and how much of it you have.