The strongest substance is diamond – it’s because the bond between carbon atoms is very strong. However, there are new materials called “nano-tubes” which use the same bond. They have very high strength and stiffness, but can be woven together to make something flexible and tough too (diamond has problems there).
Nanotubes can also be very resistant to fire. The problem we have is making something out of the nanotubes that you can make houses, cars, planes etc from. At the moment, we can only make very small amounts of these materials.