A hardwood subfloor is vital to give a good surface for the hardwood floor. The subfloor itself doesn't have to be made of hardwood.
The ideal subfloor is Douglas fir. It should be boards that are nailed down at 90 degrees to the joists. Having them at an angle holds everything much more firmly. You should use sold #2 or a better grade of Douglas fir.
Frame grade pine boards also make a good hardwood subfloor. Like fir, the wood is tight in grain. In both cases the boards should be 1 inch by 6 inches.
Plywood is the most common hardwood subfloor. It’s cheaper than boards and quicker to install. The best plywood subfloor will be ¾ inch thick. This will give ample strength to support the floor and people. A Sturdi Floor plywood subfloor is recommended most highly by hardwood associations.
It’s followed by ¾ inch OSB, or oriented strand board, plywood. Like all subfloors it should be attached at 90 degrees to the joists for strength and tightness.
Among the recommended subfloors, the lowest on the list is 5/8 inch plywood. This is cheaper but not as thick and, therefore, not as strong as a hardwood subfloor.