The highest tensile-strength material I can find reference to is either graphene or kevlar, depending on the metric. The highest known compressive strength is harder to find, but seems to be Tungston. High-entropy alloys have some extremely impressive properties in many areas. It's almost certain that even stronger materials remain undiscovered.

My question is, does there exist some theoretical hard ceiling on the strength that normal mater can reach? In the same way that nothing can move faster than the speed of light, does some physical law or process- the nature of how electron bonds work, or some quantum process at high pressures and densities, something like that- place an absolute limit on the strongest possible substance? And how strong are known materials compared to these limits?