Salomon Malmute is probably the stiffest "soft" boot out there. Closest you can get to a hard boot without actually getting a hard boot. The F22's are a step under the Malmutes.
Ride Insanos and Ride RFLs are both very stiff. I own the RFL's and LOVE them.
Burton Drivers seem to have a pretty good reputation as a real stiff boot. I don't have good experiences with Burton boots to recommend them myself.