It would depend on how you define "strongest" in your question. If you mean the hardest material, then I believe it would be the diamond. But in terms of strongest fibers in terms of tensile strength, it would have to be the Carbon Nanotube Superthread made of carbon nanotubes & polyvinyl alcohol that is 17 times stronger than Kevlar (the one used in bulletproof vests) and 4 times stronger than spider webs.
