If you plan to do your own heat treating and you don't have access to expensive high-fallutin' temp controls, then ask your buddy for some L6. If you plan to send it out for HT or if you have the high-tech stuff, then any of them will be dandy.
With proper HT, L6 will have better impact resistance than the other two. With proper HT, D2 will have better wear resistance than the other two. With proper HT, A2 will have better wear resistance than L6 and better impact resistance than D2.
Without proper HT, rebar or railroad spikes would be just as good as any of them.