Aircraft builders use this information.....and all agree that Sitka spruce has the best strength to weight ratio.
The units for stiffness are lb/in
2. (modulus of elasticity)
The units for weight are lb/ft
3.
If you divide strength by weight, you get some number per inch (or foot).
Other woods that have good ratios: Douglas fir, White pine, and Western hemlock.
That said......did you want this info so you could build the best bow?