Fire/Poison
I'm not convinced that Steel/anything is a bad typing, and Steel/Ice just screams "DRAGONSLAYER".
Meanwhile, Ice/Rock is too predictable. You can't make a defensive or even a tankish mon with that typing and have it be OU. You just can't.
With Fire/Poison, there's a ton of places this CAP can go. "Bad", perhaps, but actually usable in a bunch of different ways.