Catek FR2 Pro EVO, only thing stiffer is plates.
Not sure if I should mention Burton C60s here since they're not made anymore, but they're right up there in the stiffness range. I've heard Diodes are plenty stiff too but fragile.
I'm pairing C60's with my new fancy pants freeride board so that shows the OP what I think are stiff bindings.
I should stop promoting them considering I'm trying to find some nearly new.