I believe this would be whatever type has the most resistances and least weaknesses, standing next to steel.
The next defensive Pokemon is Rock type, which has 8 resistances (2 less than Steel) and 4 weaknesses (1 more than Steel).
ground/flying or water/ground. ground/flying has a resistance to fighting and immunity to ground, the 2 most common and powerful type moves. its also immune to electric but just watch for ice moves
water/ground is good because it only has 1 weakness which is grass and those types arent used too much.
Why didn't anyone say Dark/Ghost?