Cr2O3. CrO isn't very stable in air (though Cr3O4 might be, I'm not sure), CrO2 takes work to make I think, and CrO3 is rather reactive, not to mention carcinogenic. ;-)
But it's not so much the Cr2O3 as it is the three-way alloy that has so much bulk resistance. All those weird atoms really break up the electron flow. (Typical stainless is 10-20% Cr, 5-10% Ni, balance Fe, with smaller amounts of Mn, Si, C, and impurities of S, P, etc.)
Tim