Lead is good because it's both heavy (specifically, its nuclei are heavy) and cheap. Other dense materials also work well. But if the X-rays come from a specific atomic transition, the best absorber may be slab of that element. read more
The most used materials to block x-rays are lead and concrete. but i am not sure which material absorbs x-ray the most. lead and concrete are widely used because they are both cheap and can block x-rays well. read more