A live whitetail deer, preferably a buck of 125" or better... :D
As for practice, whatever cube/block taget is cheapest when I make a Cabelas run. When used as a dedicated broadhead target, the most expensive on the maket has about the same usuable life (actually quite short) as the cheapest, so I don't see any sense in sinking big bucks into them. You can make them last a bit longer by shooting all sides, but once that hole gets blown in the center, they aren't worth much.
The last couple 3D deer targets I've gotten were Deltas ( I think they were backyard or treestand bucks) with replacable core. When I get a target like this I buy at least one extra core. I've found if I limit broadheads to maybe 20% of the shots on this type of target, I can get 2-3 years out of a core...
If you want foam targets like these to last, particularly 3D's, don't leave them out in the weather. When you get done shooting, store them indoors. I hang mine in the barn, as if I leave them on the floor the cats use them as scratching posts...