Australian Jarrah definitely ranks up there, I think what was reffered to as Australian Desert oak may also be known as Australian Buloke or Bull-Oak. Some tout it as being part of the ironwood strain which it may be here is an interesting video of a guy explaining some of the harder and softer woods as per his research he explains of a few of the many variables of wood densities and hardness testing and why one report may greatly differ from another testing report

Another chart
Janka wood hardness scale - photo-charr.jpg
Here in Texas and some other states a known very hard wood is called Bodark, horse apple hedge apple ETC. once dried the best way to cut it is to not even try but it is no way one of the hardest woods, just like one species of Texas Mesquite at varying stages of dryness or ageing at times it is like cutting concrete with a chainsaw while at others a good sharp axe will slice through it knocking out fist sized chips