We recently came around a tree of Alpine Silver Fir that was unusual big for this species and we cut some guitar tops from it. The result was simply astonishing: this is the stiffest wood we have ever had. The taptone is very nice and loud, the top vibrates strongly.
Looking like spruce but botanically not being a spruce species, this is a real fir. Do not mix it up with the quite soft Douglas Fir from the US (what is botanically in fact not a real fir), this is a different botanical species and - again - super stiff.
It is rare and very limited, we do not know if we can get another tree that is sized big enough for guitar tops in the foreseeing future.
Below is a sample pic from this stuff, for more click here: