I know not what blade steel is best for World War III, but for World War IV Obsidian is highly recommended  
Oh  goody! Now we can talk about knappable stone,something that I as an archaeologist know something about...Obsidian, or volcanic glass,is  fairly rare, but the coming unpleasantness will produce lots of its nuclear equivalent, fused glass.  Just avoid any of the stuff  that glows in the dark...
Obsidian produces extremely sharp edges, but they are very fragile, so there will be a trade quickly developed in varieties of chert and flint (like Alibates Flint in Texas) that were widely distributed prehistorically and that produce somewhat more durable edges.
When WWIV comes, one does want to be caught knapping....