Not the hardest. Bit less than Tungsten so pretty hard but far from hardest. It is used because it is really really dense and hard enough. Perfect for piercing armor.
Depleted uranium also has a few other wonderful qualities - one is that it's self-sharpening, as the nose of the projectile will shear instead of deforming [0]. Another is that uranium burns in such situations, which means you get 'incendiary' for free.
Artificial diamond? But it doesn't matter - you don't actually want something hard because you don't actually want it to go all the way through and out the other side. You want it to dump all that lovely kinetic energy into whatever it's hit.
Yeah, DU has an incendiary property along with a "self sharpening" property. It doesn't mushroom as much when used as a sabot round as tungsten. My platoon sergeant in the US Army was a former tanker and we learned all about it (I flew the Shadow 200 TUAV).
