Fire/Poison
Rock/Ice is actually pretty well suited to the concept, the only problem is that it really limits our direction if we actually want to create an OU-viable mon - its weaknesses to so many extremely common types combined with its fantastic offensive STABs means that it is eminently well suited for a hit-and-run attacker role, and little else. It just seems very generic and doesn't exploit the potential of this concept at all.
Fire/Poison, on the other hand, can be a typing that lends itself to being a solid tank, offensive sweeper, defensive pivot, or even a supporter. We are not constrained by it, because while it's bad, it's not so defensively or offensively shitty that we are shoehorned into a playstyle. That is why it's my choice.