What is the best wood for axe wedges?

Well the title pretty much says it all. I have a good friend that is a black smith, and makes quite a number of axes. So much so the local supply of wedges have been depleted and the quality of said wedges is lack luster at best. So I have been asked to make a couple hundred. What would be a good wood to make these from? Ash, oak, walnut, popular? Any advice? Don’t want to put a crummy wedge in a $200 forged axe.

Thanks!

-- Im bleeding, go get my super glue and roll of black tape....