[vladgiverts]

I'm pretty sure Tungsten and borated polyethylene are best at shielding against different types of radiation. Dense materials like tungsten and lead are ideal for blocking gamma rays and x-rays, while borated polyethylene is better at blocking neutrons.

Depending on your environment and the sensitivities of what you're shielding, you might need one or the other, and possibly both.

Disclaimer: I'm not a nuclear physicist. I learned this while researching for a hard-sci-fi novel I'm working on.