That's a good question. Here, in central British Columbia, we have a lot of choice, but there is a BEST wood all the same. Cottonwood. These trees grow as large as five feet through the stump, are very open grained and 'soft' without a lot of knots. We cut our rounds a minimum of fourteen inches thick, nail two support poles or dimensional lumber to the back, and where those meet at the top, a third piece acts to hold the block at an appropriate angle. Off the ground, this wood lasts for twenty years or more, and cutting it thick reduces the tendency for the wood to check (split). But we use spruce and fir as well, though they are not nearly as large in diameter - perhaps as much as 28" or so. What you need is a block of end grain, as a 'hawk and knife stick so much better than into a plank. We try to avoid blocks with a lot of knots, which will spit out a tomahawk or knife in the most frustrating way.
The knife and 'hawk trail at our club has 22 targets, and most are of spruce in the 15" - 30" range. They've stood up for close to thirty years, and are attached to living trees along our trail. Again, anything on the ground returns to the earth very quickly.
