I am not sure what is the stiffest ski now is, but I would say the stiffest thing I ever skied was the K2 VO Slalom, it was a 2x4 with edges. One person alone could not flex it. I think whatever materials K2 used for it went into the formula for Viagra.
Stiffest ski is absolutely a custom flexing (or not flexing) iggy.
But stiffest "production" ski would prolly be one of the following:
-Rossi Teamstock XXXX (not the XXX Harrisons)
-Legend pro??? -I would hesitate to put my one-ski-quiver ski up there with these two by fours of yore.... it seems to versatile to be that much of a plank