Your question does not have a definitive answer, as the lightest material known is far from the strongest. On the contrary, there is a correlation between density and strength, meaning that in general denser materials are stronger than those that are less dense.
You might want to look in to Ashby diagrams, a plot where two properties are plotted against each other for many materials. In this case a Strength-Density-Diagram would be advisable.
On this site, you can find a variety of these plots.
A second more "numeric" might be to look up the breaking length of materials, which is a theoretical calculation for how long of a bar you could make out of a certain material until it fails under its own weight. There are some astonishing numbers.
The calculation is pretty easy,
You get the force exerted by the bar itself $F_T=\rho\cdot A\cdot g\cdot L$, where $\rho$ is the density, $A$ is the cross-sectional area, $g$ is the gravitational acceleration and $L$ is the length of the bar.
On the other hand you get the maximum force the bar can support $F_R=R\cdot A$, where $R$ is the maximum bearable stress (i.e. ultimate tensile strength, UTS).
If you set the forces equal you get
$$ F_T=\rho\cdot A\cdot g\cdot L = R\cdot A = F_R$$
As you can see the area cancels out and if you solve for $L$ you get
$$ L=\frac{R}{\rho\cdot g} $$
which means the breaking length of a material is only defined by its UTS and density (assumed $g$ is constant).
There's a neat table on Wikipedia on breaking length / specific strength (which is just $=\frac{R}{\rho}$) but there surely are other sources as well.
